I think the exact answer would be : The fall of Bastille was due to the rumours that were spread that the king Louis 16 would soon order the army to open fire upon the citizens. Yet the Bastille was hated by all because it stood for the despotic power of the monarch.
Second reason can be the subsistence crisis during old regime. The high prices of bread and low pace in the harvest.
The third reason can be the division of the French Society into three estates FIRST ESTATE(CLERGY), SECOND ESTATE(NOBLES) AND THIRD ESTATE(LANDLORDS, PEASANTS, ARTISANS, etc.)