At the end of WW2, multiple German leaders either committed suicide or made preparations to do so -- was this unprecedented?

There was one important difference between Nazi leaders and other defeated leaders that (probably) made many Nazi leaders commit suicide. That is the fact that they had committed "war crimes" against civilians, and realized that they would be charged with that. That was what was "unprecedented."

Shortly before his execution at Nuremberg, Hans Frank, the governor of the "General Government" (occupied Poland) said the following: "A thousand years will pass and still this guilt of Germany will not have been erased."

In earlier times, the casualties of wars, while severe, were mostly limited to fighting men. There wasn't an idea of "total war" against everyone on the other side, including unarmed civilians. In his time, America's William T. Sherman perhaps violated this taboo by waging war against Confederate civilians, but he "only" threatened their property, and not their lives.

Leaders who had been defeated but who had conducted themselves "honorably" (for their times) were usually not executed and had no reason to commit suicide.

It certainly wasn't unprecedented - plenty of defeated combatants kill themselves to avoid capture, rape, or humiliation. Examples:

Emperor Chongzhen of the Ming Dynasty

Word reached Beijing that Shun rebels were approaching the capital through Juyong Pass, and the Chongzhen Emperor held his last audience with his ministers on 23 April. Li Zicheng offered the emperor an opportunity to surrender, but the negotiations produced no result. Li commanded his forces to attack on 24 April. Rather than face capture by the rebels, the Chongzhen Emperor gathered all members of the imperial household except his sons. Using his sword, he killed Consort Yuan and Princess Zhaoren, and severed the arm of Princess Changping.

On 25 April, the Chongzhen Emperor was said to have walked to Meishan, a small hill in present-day Jingshan Park. There, he either hanged himself on a tree, or strangled himself with a sash. By some accounts, the emperor left a suicide note that said, "I die unable to face my ancestors in the underworld, dejected and ashamed. May the rebels dismember my corpse and slaughter my officials, but let them not despoil the imperial tombs nor harm a single one of our people."

The Japanese practice of Seppuku.

As a samurai practice, seppuku was used voluntarily by samurai to die with honor rather than fall into the hands of their enemies (and likely be tortured) ...

At the end of the Siege of Masada.

The ramp was completed in the spring of 73, after probably two to three months of siege. A giant siege tower with a battering ram was constructed and moved laboriously up the completed ramp, while the Romans assaulted the wall, discharging "a volley of blazing torches against ... a wall of timber", allowing the Romans to finally breach the wall of the fortress on April 16, 73 CE. When the Romans entered the fortress, however, they found it to be "a citadel of death." The Jewish rebels had set all the buildings but the food storerooms ablaze and had killed each other, declaring "a glorious death ... preferable to a life of infamy."

Aftermath of the Jingkang Incident:

Emperor Taizong feared that the remaining Song troops would launch a counter-offensive to reclaim the capital. Therefore, he set up in Bianjing a puppet government for the lands south of the Yellow River, called Chu (楚), and ordered all the assets and prisoners to be taken back to the Jin capital – Shangjing (in present-day Harbin). The captives marched to the Jin capital along with the assets. Over 14,000 people, including the Song imperial family, went on this journey. Their entourage — almost all the ministers and generals of the Northern Song dynasty — suffered from illness, dehydration and exhaustion, and many never made it. Upon arrival, each person had to go through a ritual where the person has to be naked and wearing only sheep skins. Contrary to what was previously thought, the ceremony was drawn from ancient Han Chinese customs, drawn together by Jin experts on China rather than a Jurchen ritual. Empress Zhu committed suicide because she could not bear the humiliation ...

Some other famous people who killed themselves to avoid capture are Brutus, his co-leader Cassius, Mark Antony and Cleopatra.

We are talking about 175-180 suicides of the Nazi leadership, from Hitler down to individual Reichstagsabgeordnete and high ranking Officers (from cursory reading: Significantly more suicides in the Waffen-SS than the Wehrmacht), some Mayors etc. There were probably thousands, maybe low ten thousands of people with similar rank and standing who did not commit suicide. It should also be noted that there was a wave of suicides in the general population around the immediate end of the war and a decision about wether a specific suicide was by a leader will always be a bit subjective.

Also even in the last days of the war, some Wehrmacht and Volkssturm units still tried to fight off the Allies, that the Nazis persecuted deserters right until the end (The judges who signed the execution warrants as a rule didnt commit suicide and often had successful post-war careers.). I would see this stubborness in fighting (and murdering, see the death marches) on even after the war was clearly lost on the same spectrum as the suicides: The inability to simply quit fighting and murdering and going into captivity.

As for reasons, Tom Au suggests persecution and the fact that the Nazis did not conduct themselves honourably by the measures of their time. I don't think that's wrong, but I would suggest another approach.

In the very militarized, apocalyptic worldview of the Nazis, there was only victory or armageddon. Their vision of total war was a war between the totality of the individual peoples. I don't think they could really conceive of a post war order, after losing their war, in which a recognizable Germany would exist.

Also, fascism is a death cult, or rather a

cult of heroism [that] is strictly linked with the cult of death. It is not by chance that a motto of the Falangists was Viva la Muerte (in English it should be translated as “Long Live Death!”). In non-fascist societies, the lay public is told that death is unpleasant but must be faced with dignity; believers are told that it is the painful way to reach a supernatural happiness. By contrast, the Ur-Fascist hero craves heroic death, advertised as the best reward for a heroic life. The Ur-Fascist hero is impatient to die. In his impatience, he more frequently sends other people to death.

(Umberto Eco: Ur Fascism)

So one could look for these aspects - apocalyptic vision, cult of death - in the last known words from Nazi leaders before their suicides.

One should also add that while the Nazis had apocalyptic visions, many of those who killed themselves could probably have had decent careers in post-war Germany, at least in the west.