To me, the reason for this is due to the Roman system of patronage. Rome's patronage system consisted of two types of people -- patroni and clientes (who could be the patronus of other men at the same time). A man's political power was directly tied to the effectiveness of his duty as a patronus -- if he was able to give help to his clientes, they would vote for him and do him other favors. The more help he could offer, the more clientes he could attract. If you anger the masses, however, they might find another patronus, thus reducing your personal power (and then your political power, since you have lost some votes).

For more information on patronage, see Personal Patronage in the Early Empire, Richard P. Saller.

While the Roman world is often made out to be a society with either patricians or plebs the political situation in the late republic was a lot more complicated. By the second century BCE there were two rival factions in the Roman upper class. A man by the name of Tiberius Gracchus was elected a Tribune of the Plebs in Rome. He used his authority in a way that very few others had. He enacted a land reform program that ensured public land for poor Romans at the expense of massive private estates held by the senatorial class. This eventually sprung a conflict between to political divisions. They were not really well defined like modern political parties but the Optimates consisted of the conservative members of the senate who preferred a traditionally powerful senate and a weak people's representative. The Gracchi were Populares who relied on the support of the people for their lower positions. The conflict between the Populares and the Optimates continued of and on until the end of the republic with many well known figures on both sides. Once more, these weren't always political parties like we know today, rather both sides had support and often threw their weight behind certain figures who elected to use separate methods for attaining political power.

When you talk about the Roman Nobel’s, in the early republic you are referring to the Patricians. Now this class is hereditary and the rank is passed from farther to son, as a result they are slowly dying out. Early on they hold a great deal of power and influence within Roman politics; however towards the period of the late republic as a class they have lost their pre-eminence. As a large number of Plebian families have gained great wealth and political influence. This results in the breaking of the powerful families in Rome, new and old into two distinct factions, these being the Optimates and the Populares. It is this war of factions that I believe best addresses your question. As during this period, 133BC-80BC, is when the Optimates, or the nobles, were most afraid of what the people thought.

The first step to understanding why they are afraid of the people is to understand the power that the people wielded. Of most note is the position of Tribune of the Plebs, this was introduced in 494BC in response to the perceived manipulation of the higher offices held by partition families. This office was a direct threat to the control that the dominate families held through the traditional magistrate offices. This stemmed from the capacity to veto pieces of legislation. Their response to this was to run candidates either from their families, or to make candidates or current tribunes their clients through giving their support. Thus they provided the office and placated the common citizens, whilst maintaining power over this office.

This was business as usual for the most part, until Tiberius Gracchus took up the position in 133bc. It wasn’t the actual piece of legislation the Tiberius was attempting to introduce that caused these fears to come into existence. The Lex Sempronia Agraria, as it was known was really just a simple reinforcement of an existing law. What made the Lex Sempronia Agraria scary for the elites was the manner in which Tiberius introduced his legislation. He simply put the bill to a public vote, thus avoiding the senate bogging down the proposal in committee and dodging its recommendation. In response to this act the societal elite did what they did when ever ‘unfavorable’ legislation presented its self. They used one of the tribunes they owned to Veto the bill. It is what happened next the truly worried the societal elites, Tiberius’ response to Octavius’ attack was to have him deposed from office and introduce additional legislation to ensure that no tribune, once deposed would be able to hold public office again. Both of these motions carried and form there, Tiberius was able to march on and pass the bill that he cared for so much.

This raises the question of why they simply didn’t have Tiberius killed and be done with the whole three act play. Well Tiberius had thought of this, and he was relying on the idea of a Tribune being sacrosanct. This means that to kill Tiberius what have been punishable by death, it was also a religious offence and would have shown a massive breakdown in the legal and traditional framework of Rome. Thus as long as Tiberius remained tribune he was safe. Up till now Tiberius had been a threat to the temporary power of enjoyed by the ruling oligarchy. However due either to fear for his life, post tribune, or because he enjoyed the power that he currently wielded. Tiberius attempted to run for a second tribunate. This was more than simply controversial, however Tiberius by this was seen as an act that could promote Tiberius to the level of a virtual Rex, or a king as we would know it, this was feared above all things in the eyes of the oligarchy. It is during a discussion as to Tiberius’ eligibility to run for reelection that the details become contested and reports vary, sufficed to say Tiberius is killed along with 300 of his supporters and is dumped into the river Tiber.

This is the first real example of the ruling oligarchs fearing what people thought and who they favored. It was Tiberius’ ability to control the tribunate as an individual through skill in oratory that scared the oligarchs and the precedent that he set leads in no small part to the downfall of the republic, as others such as his brother, Sulla and Marius all abuse the power that Tiberius has shown this office could hold.

I realize this is have omitted several details, including those of the Lex Agraria, previous legislation, the details of Tiberius’ death and most of all how others used this precedent. However this post is long as it is and does serve to answer the question at a basic level. If there is an interest, I would be more than happy to elaborate on how this president was employed, and this damage to the foundation of the republic exacerbated.