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.