what stopped them from acting against the 'will of the people'?

Is that really what the tribunes were for? In concept at least the tribunes existed as a senatorial vehicle for the plebs to protest coercion and the violation of their rights. Livy mentioned (technically in a passage referring to the early Republic, but like much of his discussion of the early Republic really anachronistically talking of his own time) that the tribunes from their very onset were the tools of the senatorial class:

quem auctorem aliquot annis ante fuisse memoratum est per collegarum intercessionem tribuniciae potestatis dissolvendae.

And he (App. Claudius) was remembered to have some years before been the originator of the idea of breaking up the power of the tribunes through the intervention of their colleagues.

As Brunt put it, "The senate could almost always find at least one tribune to act on its behalf." Whether this loyalty was secured through bribery, promises of political support, or simply the fact that the tribunes were senatorial themselves depends on the individual. M. Octavius, for example, famously vetoed Ti. Gracchus' motion for land distribution, and had his magistracy abrogated under questionable circumstances. Why? Plutarch suggests that Octavius possessed large quantities of the ager publicus, and that therefore he was acting in his own interest and the interests of landowners like himself. Ti.'s brother C. Gracchus had similar problems with his own tribunician colleague Livius Drusus. Plutarch and Appian both are of the opinion that the senatorial class induced Drusus to oppose Gracchus for their own reasons. But Drusus is an indicative example. The plebs were not a united group--even within the city hundreds of little subgroups dominated according to their residence, station of life, and local origin. Throughout Italy Roman citizens of every class lived and held political opinions, which often differed quite strongly from those of the cityfolk. In 89 Sulpicius Rufus proposed a bill distributing the newly-enfranchised Italians into all the voting tribes, thereby securing for them a position of equal electoral power. They were violently opposed, says Appian, by the οἱ ἀρχαιότεροι, "the original citizens," which in context and in comparison with similar parallels at other times must almost certainly refer not only to the nobility but to the urban plebs themselves, who were distributed into only four tribes and would suffer an even greater loss of electoral power if Sulpicius' bill went through (as it did). Likewise Livius Drusus: Drusus proposed the foundation of twelve colonies over Gracchus' two, along with some measures directed towards the Latins. The establishment of colonies was generally understood as an urban gesture, as opposed to the distribution of the ager publicus, and Appian considers it so important as to omit Drusus' other legislation. Drusus' son, also Livius Drusus, similarly may have attempted conciliation of the urban plebs by providing them with a grain bill^^1

Likewise, the holder of the tribunate need not necessarily maintain himself as a "populist" at all. His duty was, at least in theory, to maintain the rights of the plebs against coercion and other outrages, but it was not necessarily to do whatever the plebs wanted--and the man who thought he could please the entirety of perhaps the most diverse class of citizens the world had then yet known was doomed, as Drusus learned the hard way. Many tribunes, however, as a part of their office, temporarily donned their popularis hats. What Tatum has dubbed "popularis tactics" were not some kind of steadfast oath to which politicians swore. The Roman world knew no sort of thing as a political platform, nor did it have political parties. An individual lent or withdrew his support for a motion or group as the opportunity presented itself and whenever he pleased. Often personal injury was enough of a reason to champion popularis action. Cn. Domitius Ahenobarbus in his tribunate in 104 was refused cooption into the college of pontiffs. His recourse was to propose a motion to the plebs that transferred election of the pontiffs to the people, which was passed and which he used to secure his own election. It took a moment of personal disgrace to push Domitius over the edge, despite the fact that as tribune logically a modern reader might expect him to have championed such a motion in the first place. Moreover, following his tribunate Domitius' career was pretty normal and ordinarily senatorial--this, indeed, was the norm for populares (at least those not murdered), who regularly withdrew back into the ranks of the senate when curule magistracies were at stake, or at the very least did not champion popularis measures so vocally and, sometimes, violently. The classic example is Cato. M. Cato as tribune in 63 passed a grain bill expanding the right of state-subsidized grain. Yet in that same year and same tribunate Cato was the author of the motion to execute the Catilinarians without trial, for which action the tribune Nepos vetoed Cicero's right to deliver a parting address when laying down his consulship. How can these be reconciled? Simply by realizing that the office of the tribune of the plebs was not necessarily "representative" in the way that we mean in modern politics--tribunes represented something, but they were not necessarily elected to be mouthpieces to all the grievances of the plebs, and indeed could not possibly have supported the entire body of plebs with all its diversity. Polybius described the tribunate as existing in order to carry out the decisions of the plebs, but whether that was really true in practice and whether the tribunes themselves necessarily should be expected to author plebeian motions themselves are rather different problems.

(Cont. below. My god I didn't want to write so much...)