Why was Scipio the Younger accused of wishing to abolish Tiberius Gracchus laws?

There is now waning consensus in scholarship on what caused the crisis in Rome after the second Punic War. But there is still consensus in scholarship as was in the res publica that a reform was desperately needed. Where this ancient and modern consensus diverges again is on how to achieve that re-organisation.

Tiberius Gracchus was staunchly opposed by Scipio in everything he did, it is said. But his younger brother Gaius sought another way to accomplish the same greater goal and on that Scipio saw himself as a sort of moderator. Scipio was still being the conservative but tried to negotiate instead of just opposing. What sounds like a great attempt of walking the middle ground turned out to be him left sitting between chairs.

But this somewhat different as compared to what is posed in the question above:

If Tiberius fought for the citizenship of Italian allies, why would the people be mad at him for standing with such allies. Even if the Roman citizens didn't want the citizenship to be extended to the allies, to accuse Scipio of trying to abolish such laws is not coherent.

It is not about gaining citizenship but what that citizenship means: the distribution of wealth, power and that means land. Who has it, who gets it, under what conditions and who decides that wit what procedures. All those Roman peasants giving up their rustic lives as farmers, still fabled and romanticised by the Roman public mind, where now forming a lackland proletarian mass. Even the owners of latifundia where in favour of giving them some land so that they would be busy cultivating it. But the majority of the senate was assuredly optimates in modern terms "conservative" and thus no fans of any change – or redistribution – and thus perhaps change at their expense.

The allies/socii were just a small factor in the thoughts. "No Roman? Don't care!" (Like the Romans talking about their allies lime in All your base are belong to us?) was the main line of thought for almost all parties involved in that discussion; and it was precisely the reason why Scipio went into the discussion with his ideas and connections.

The Gracchi wanted to give land to the masses. Assuming that Gaius might (this is unclear among researchers) have even promised some extension of citizenship: this is then to gain more support for his reforms, not the goal of the reforms. It was only a means to an end as he needed more support and popularity, which he would have calculated to gain from such extension.

It it therefore a point described in the quote from Wikipedia/Appian that Scipio was first seen as taking the side of the wealthy in opposing Tiberius, then tried to appease both sides in acknoowledging the need while still opposing the procedure and trying to find a way, but when public tended see their solution to further disenfranchise the allies in order to do so, the his words of caution where re-painted by both sides who did not want a compromise. Democratic debate and politics of the finest kind… Both parties saying that "there is no alternative".

The Senate pressed home its advantage: it set up a court […] to punish more of Gracchus’ surviving supporters. Many were condemned and executed. […] Despite this display of strength the Senate decided that, since Nasica’s continued presence in Rome would remind meny of his violation of the sacrosanctity of a tribune, he was better out of sight: he was therefore sent on a commission to Asia, where he soon died.

The Senate, however, did not interfere with the working of the agrarian commission, a fact that demonstrates that its objection to Tiberius’ bill was much weaker than its dislike of his methods. Tiberius’ place on the commission was taken by Gaius Gracchus’ father-in-law, P. Licinius Crassus, but as consul in 131 Crassus secured by intrigue a command in Asia, where he died in 130; Ap. Claudius, the other triumvir, also died. Their places were filled by M. Fulvius Flaccus and C. Papirius Carbo, who, together with Gaius Gracchus, remained in office until 122. As tribune in 131 (or 130) Carbo carried a measure to extend secret ballot to legislative assemblies of the People and proposed one to legalize re-election to the tribunate.

Scipio Aemilianus, who was back from Spain after sacking Numantia in 133, helped to defeat the proposal, though a similar measure possibly was carried soon after his death in 129. The turbulence of the times was reflected during the censorship of 131, when this office was held by two plebeians for the first time in history; a disgruntled tribune tried to push the censor Q. Metellus Macedonicus over the Tarpeian Rock, from which condemned criminals were hurled.

More memorable perhaps was Metellus’ speech ‘de prole augenda’, an appeal to enforce marriage and thus increase the birth-rate; he was clearly conscious that Rome’s economic difficulties required reform, though his solution was on different lines from that of Gracchus.

Meantime the commissioners were hard at work, asserting the State’s claim to the extra ager publicus and distributing it to new settlers.

But difficulties arose, especially where the interests of the Latin and Italian allies were involved. Already exasperated by Rome’s recent treatment of them, those allies who had been allowed to occupy ager publicus in the past would not now enjoy having to surrender any surplus they held in order to provide allotments for the unemployed from Rome, while disputes may have arisen with the commissioners over the title to borderland where land originally taken from the ally by Rome ran alongside land retained by the ally.

To air their grievances they sought the help in Rome of Scipio Aemilianus who as a soldier knew the true value of the allied contribution to Roman life. In 129 their new patron persuaded the Senate to warn the commissioners not to deal with disputes about land held by the allies; such cases should be transferred to the consul Tuditanus, who conveniently went off to Illyricum. But the distribution of land went on: the census figures of 125 BC (about 395,000) were some 75,000 higher than those of 131 BC, and this increase almost certainly reflects the progress of land settlement.

Scipio’s championship of the allies increased his unpopularity with the urban mob who had already disapproved of his opposition to Carbo’s bill. One morning, when he was due to make a speech on the Italian question, he was found dead. His death remained an unsolved mystery. Although various people were blamed at the time or later (including Flaccus, C. Gracchus, Carbo, and even Sempronia or Cornelia), murder is not very likely; suicide is possible, but most probably he died a natural death (a heart-attack?). Rome thus lost an upright soldier, who had exercised a moderating influence on her political and cultural life.

H. H. Scullard: "From the Gracchi to Nero. A history of Rome from 133 BC to AD 68", Routledge: London, New York, 2011. (p 25–27.)