The root causes of these struggles were the desire for more power on the part of various parts of the society and the decline in the number of small family farmers.
The first of these problems was caused by the division of the aristocrats between the optimates and the populares. These were, respectively, people who wanted the elites of the Senate to retain control of the government and those who wanted more of a voice for the plebs.
At the same time, there were problems with the decrease in small farmers. This was going on for a variety of reasons, and it had an important impact. Many people who had once been small farmers started to move to the city. They formed an unstable urban proletariat that could be a real problem given the right circumstances.
These problems came to a head in 133 BCE when the Gracchi came to power.