Was it a multitude of reasons having to do with the grain shortage, the growing distrust of the nobles, or even the printing press making it easier to spread ideas of revolution?
The economic situation in France had been rapidly deteriorating in the years leading up to 1789. If asked for the "main cause" of the French Revolution, many would tell you it stemmed from this. France had been involved in two major wars, which, when compounded with several years of drought, put a great strain on the economy. As is typical, the lower classes were most hardly hit by the situation.
It got bad enough that Louis XVI called the Estates-General of 1789 to discuss solution for the economic issues troubling France. Three Estates were established, The First, Second, and Third. Clergy, Nobles, and Commoners, in that order. After months of unsuccessful meetings, the Third Estate formed the National Assembly, effectively beginning the Revolution.
As is typical with revolutions, the French Revolution was caused by the hunger of the commoners. There were several factors that compounded to the situation, but the underlying factor is that a large percentage of the population was faced with massive food shortages.
Please note that this does not address what caused the French Revolution gain as much momentum as it did. After this initial spark, there were a multitude of factors that contributed toward the intensity and the direction of the Revolution.
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The impositions on the lower classes are well-known, but there is a reverse side to this.
The French ruling classes repeatedly blocked any attempts to spread the burden onto them. Louis XVI's finance minister, Necker, tried repeatedly to influence the nobles and clergy to bear some of the strain, but they used their influence to block this until it was too late. Louis is often described as a 'weak' king and his failure to secure this compromise is a key reason for this reputation.
Another aspect is the stifling by the ancien regime of the emerging middle class (a stark lesson in light of the 'Arab Spring'). Virtually all the influential figures of the revolutionary period and most important deputies elected to represent the Third Estate at the the Estates General were what we would now call middle class professionals: merchants, lawyers and so on. Whereas the British had managed to reconcile the interests of the aristocracy and the wealthy people not of 'high birth', the French system built up by Louis XIV and Richelieu was designed to entrench royal power... which just mean the collapse was later and more violent. Such people cannot be excluded from the political system and to an extent the early path of the Revolution was their way of taking power in the name of the people. Institutions like the Jacobin club and Freemasons lodges were the vehicles by which these groups met and planned.
The violence of the Revolutionary period is often discussed in terms depicting or implying it was an outrage, unnecessary, and as a warning against radical politics. But the sheer intransigence of the old order and the unwillingness of the old rulers to compromise is, in my view, the major contributor to the violence. The threat to the Revolution from a counter-revolution was very real.