I am going to add also that the atmosphere of the French Revolution had, as a backdrop, the philosophical influences that are unique to the Age of the Enlightenment. Although it was not the Enlightenment what caused the revolution, there is certainly enough influence from the writings of Rousseau, Voltaire, and Montesquieu that did influence many of the persons who contributed to it, or participated in the revolution. It is in the writings of these three philosophers, among others, where we can find ideas such as the division of power and the end of the absolute sovereignty.
Like most revolutions, different people wanted different things out of it. The aforementioned peasants generally wanted to relieve themselves of some of the feudal restrictions that they still labored under, and to be able to keep some of the fruits of their labor. The ergot theory has been widely discredited--the peasants knew what they were doing. Urban workers--the sans-culottes and below, demanded economic measures to better their lives, which were subject to not just fluctuating grain prices, but many technological changes that were beginning to affect skilled craftsmen. As the previous sentence indicates, there were even divisions within the urban working class The men we usually associated with the Third Estate--merchants, lawyers, etc.--wanted an end to many of the onerous taxes as well as the unfair privileges afforded the nobility, which they came to see as useless and parasitical. Many other educated Frenchmen wanted liberal legal reforms, including basic civil liberties protections and constitutional limits on the monarch. These few examples are a very rough sketch of a Revolution that has bedeviled attempts at a single interpretation since Carlyle, Tocqueville, Constant, and others tried to make sense of it in the early nineteenth century.
With the increase of merchants and lawyers who were relegated to the Third Estate (many of whom were Jews), there was from them a great desire to elevate themselves to have some voice in the French government. (This situation is not unlike that of Russia before its revolution.) They became activists for legal voices, and probably incited the already disgruntled peasants who paid taxes on the land that they worked for the aristocrats.
Curiously, there have been scientific investigations that have uncovered the fact that the grain crop that was so bad in 1789--bread being the staple of the poor, especially--caused, not only famine, but a mold on the bread made from this crop. Some evidence points to this mold having caused hallucinogenic qualities, so there is a theory that the peasants were on some kind of "trip." Whether this is true or not is certainly debatable; however, it is certainly odd that people would all react on one particular day and revolt. This is an idea that Leo Tolstoy considers in his War and Peace as he reflects upon how an old general who many times was distracted was able to defeat the mighty Napoleon at war. Tolstoy theorized that there is some cosmic force in place in history.
To learn more about the French Revolution, you may wish to examine Thomas Carlyle's The French Revolution, a history that greatly influenced many, among them Charles Dickens, who wrote A Tale of Two Cities, based upon this revoltuion.
For most of the people who participated, the motivation was a desire for more of a say in their lives. The people of the Third Estate essentially had no power in the French government. They ended up paying all the taxes without having any control of their government. This injustice helped to motivate them to participate in the Revolution.