Why is there a big gash in the side of the Pyramid of Menkaure?

Per the Wikipedia article:

At the end of the twelfth century al-Malek al-Aziz Othman ben Yusuf, Saladin's son and heir, attempted to demolish the pyramids starting with Menkaure's pyramid. The workmen who Al-Aziz had recruited to demolish the pyramid found it almost as expensive to destroy as to build. They stayed at their job for eight months. They were not able to remove more than one or two stones each day at a cost of tiring themselves out utterly. Some used wedges and levers to move the stones while others used ropes to pull them down. When it fell it would bury itself in the sand requiring extraordinary efforts to free it. Wedges were used to split the stones into several pieces and a cart was used to carry it to the foot of the escarpment, where it was left. Far from accomplishing what they intended to do they merely spoiled the pyramid by leaving a large vertical gash in its north face

Sources for the Wikipedia section are:

• Stewert, Desmond and editors of the Newsweek Book Division "The Pyramids and Sphinx" 1971 p. 101

• Lehner, Mark The Complete Pyramids, London: Thames and Hudson (1997)p.41 ISBN 0-500-05084-8

The page on ben Yusuf states

During his reign, he tried to demolish the Great Pyramids of Giza, Egypt, but had to give up because the task was too big. However, he did succeed in damaging Menkaure's Pyramid.

The source for that is Egyptologist.org, which goes into great detail on the damage. An excerpt:

When king Al-Aziz Othman, son of [Saladdin] succeeded his father, he let himself be persuaded by some people from his Court, who were devoid of good sense, to demolish the pyramids. One started with the red pyramid, which is the third of the great pyramids, and the smallest. So the sultan sent sappers, miners and quarrymen, lead by some of the main officers and the first emirs of his Court and ordered them to destroy it.... This happened in the year 593 (i.e. 1196 A.D.)." (transl. SACY, Description de l'Egypte IX, 468).

He cites Description de l'Egypte IX, 468, which is in French. Description de l'Égypte is a "series of publications, appearing first in 1809 and continuing until the final volume appeared in 1829, which offered a comprehensive scientific description of ancient and modern Egypt as well as its natural history."

Passage from Description de L'Egypte:

Note that this section of Description de L'Egypte, de Sacy is actually excerpting his own translation of 'Abd al-Latif al-Baghdadi's Account of Egypt; so the credit for the tale should be given to him. Al-Baghdadi lived contemporaneously with these events; in fact, Al-Aziz Uthman, the sultan in the tale, was his patron at one point. (Though presumably the account below was promulgated after he left al-Aziz's patronage, given how poorly the story reflects on al-Aziz.)

Please feel free to improve on my English translation below, or (if you're feeling ambitious) dig up al-Baghdadi's Account of Egypt and translate it directly from the Arabic.

Quand Mélic-alaziz Othman ben-Yousouf eut succédé à son père, il se laissa persuader par quelques personnes de sa cour, gens dépourvus de bon sens, de démolir ces pyramides ; et l'on commença par la pyramide rouge, qui est la troisième des grandes pyramides et la moins considérable.

Le sultan y envoya donc des sapeurs, des mineurs et des carriers, sous la conduite de quelques-uns des principaux officiers et des premiers émirs de sa cour, et leur donna ordre de la détruire. Pour exécuter les ordres dont ils étoient chargés, ils établirent leur camp près de la pyramide; ils y ramassèrent de tous côtés un grand nombre de travailleurs, et les entretinrent à grands frais. Ils y demeurèrent ainsi huit mois entiers, occupés avec tout leur monde à l'exécution de la commission dont ils étoient chargés, enlevant chaque jour, après s'être donné bien du mal et avoir épuisé toutes leurs forces, une ou deux pierres. Les uns les poussoient d'en haut avec des coins et des leviers, tandis que d'autres travailleurs les tiroient d'en bas avec des cordes et des câbles. Quand une de ces pierres venoit enfin à tomber, elle faisoit un bruit épouvantable, qui retentissoit à un très-grand éloignement, et qui ébranloit la terre et faisoit trembler les montagnes. Dans sa chute, elle s'enfonçoit dans le snble; il falloit derechef employer de grands efforts pour l'en retirer ; après quoi, l'on y pratiquoit des entailles , pour y faire entrer des coins: on faisoit ainsi éclater ces pierres en plusieurs morceaux; puis on chargeoit chaque morceau sur un chariot pour le traîner au pied de la montagne qui est à peu de distance, et où on le jetoit.

Après être restés longtemps campés en cet endroit, et avoir consommé tous leurs moyens pécuniaires, comme leur peine et leurs fatigues alloient toujours en croissant, que leur résolution au contraire s'affoiblissoit de jour en jour, et que leurs forces étoient épuisées, ils furent contraints de renoncer honteus*m*nt à leur entreprise. Loin d'obtenir le succès qu'ils s'étoient promis, et de réussir dans leur dessein, ils n'en retirèrent d'autre avantage que de gâter la pyramide, et de mettre, dans une entière évidence, leur impuissance et leur faiblesse. Ceci se passa en l'année 593 [1196]. Aujourd'hui, quand on considère les pierres provenues de la démolition, on se persuade que la pyramide a été détruite jusqu'aux fondements; mais si, au contraire, on porte les regards sur la pyramide, on s'imagine qu'elle n'a éprouvé aucune dégradation, et que d'un côté seulement il y a une partie du revêtement qui s'est détachée.

English Translation:

When Mélic-alaziz Othman ben-Yousouf succeeded his father, he let himself be persuaded by some people in his court, men without good sense, to demolish these pyramids; and they started with the red pyramid, which is the third of these great pyramids and the smallest.

The sultan therefore sent sappers, miners, and workers, led by some of the main officers and leading emirs of his court, and ordered them to destroy it. To execute the orders with which they were charged, they established their camp near the pyramid; they gathered from all around a large number of workers, and maintained them at great expense. They stayed there eight whole months, with all hands occupied with execution of the commission with which they were charged, removing each day, after inflicting much pain on themselves and exhausting all their strength, one or two stones. Some pushed from above with wedges and levers, while others pulled from below with cords and cables. When one of the stones finally fell, it made a dreadful noise, that resounded at great distance, and which shook the ground and make the mountains tremble. When it fell, it buried itself in the sand; it was again necessary to make great efforts to pull it out; after which they made notches in the stone to insert wedges: they thus managed to break these stones into many pieces; then they loaded each piece on a cart to take it to the foot of a mountain a short distance away, where they discarded it.

After having stayed encamped in this place for a long time, and having spent their pecuniary resources, with their pain and fatigue always growing while their resolve weakened day by day, and with their strength exhausted, they were forced to shamefully give up their enterprise. Far from gaining the success they were promised, and accomplishing their design, they got nothing from it but to mar the pyramid, and to show, in full evidence, their impotence and their weakness. This happened in the year 593 [1196]. Today, when one looks at the stones coming from the demolition, one can persuade oneself that the pyramid was destroyed down to the foundations; but if, in constrast, one looks at the pyramid, it seems to have suffered no degradation, and that on one side only a part of the facing has come off.