One of the grand properties of a mob is that mobs will often riot, regardless of what all-encompassing idea they may or may not have in their heads. See, a mob wants to fuck shit up, and really, when it comes to the actual upwards shit-fucking, one reason is as good as any other. This can really help out those who have an agenda... get the mob to sign off on your ideals and none can stand in your (mob's) way.

The leaders of the newly ascendant Third Estate were not this canny (or psychic, for that matter). But they were lucky. The mob that freed the Bastille Seven didn't want to admit that most of their anger was fueled by high bread prices. So they picked up the revolutionary drumbeat that was first sounded in a Versailles tennis court not a month before.

The rest of the story : the Kingdom of France was well and truly screwed in the spring of 1789. They were screwed for a multitude of reasons, but it all came down to cash in hand - the state coffers were nearly empty. (And the term 'deficit spending' hadn't been invented yet, much to Louis XVI's chagrin.) So, in an effort to effect enough reforms to help fix the cash flow problems without wrecking the feudal system currently in place, an Estates-General was called in April. In the body, 300 members of the nobility, 300 members of the clergy, and for the commoners... 600 representatives. This was a concession from the King, one that he doubtless considered to be an empty gesture; after all, in an Estates-General, each Estate votes as an equal bloc. While the numbers of the the Third Estate were definitely gaudy (and meant to signal greater importance), anyone with control of the majority of the first two houses should get his way, and the King had that.

The doubling of the numbers, that was the inch; here's the mile. From Day One, the Third Estate demanded that voting switch to a per-member basis, thereby creating a new majority - the reform-minded minorities in the First and Second Estates with the whole of the Third Estate. No discussion would go forward until this issue was resolved, and it wouldn't get resolved - Louis XVI wouldn't give up a sure thing that easily. One thing led to another, and on June 20, 1789, in an attempt to learn the gridlocking commoners some manners, the doors of the posh Assembly Hall in Versailles were locked.

Bad move. Left to their own devices, the members of the Third Estate began agitating. They took over a nearby tennis court and all members were given the following oath :

The National Assembly, considering that it has been
summoned to establish the constitution of the kingdom,
to effect the regeneration of public order, and to 
maintain the true principles of monarchy; that nothing
can prevent it from continuing its deliberations in 
whatever place it may be forced to establish itself; 
and, finally, that wheresoever its members are 
assembled, there is the National Assembly;

Decrees that all members of this Assembly shall 
immediately take a solemn oath not to separate, and to 
reassembly wherever circumstances require, until the 
constitution of the kingdom is established and 
consolidated upon firm foundations; and that, the said
oath taken, all members and each one of the 
individually shall ratify this steadfast resolution by
signature.
And so was born the Oath of the Tennis Court. All present members ratified it, save one; the odd man out almost got his ass beat. (Good timing on his part - if he had dared to stand up to the crowd six years later, he would have been hanged.)

Now let's be honest; this was somewhat revolutionary, as it was the first statement of sovereignity by the people (weak as it was), and it was a good show; the King relented and formed a National Assembly in the beginning of July. And with his hand in it again, Louis XVI might've found a way to come out on top. After all, King Louis had the momentum; centuries of monarchial rule is hard to argue against. But he was also dousing little candle-flares of riots the country over, and eventually one burned too bright to be ignored.

And so the National Assembly, by dint of timing and good luck, rode the back of the Parisian mob into high standing. All revolutions use violence as a fuel, and the triumph of the Third Estate could not (and, later, would not) have been completed without the storming of the Bastille. So while this event is held to be seminal, it only is in retrospect.

But it was celebrated in the day, due mostly to propaganda of the leaders of the Third Estate. They took full advantage of the situation to claim a major coup, and they added rose tinting to the tennis court scene itself. This romantic coloring was achieved with the help of the great artist, revolutionary, and propagandist, Jacques-Louis David. His chairoscuro take on the proceedings of the day were oft-printed, to the delight of those who took the oath, as it cast them all in a fine, upstanding light. David did not witness the Oath itself, and he couldn't have cared less; he was raised on gloriously clean reproductions of Greek and Roman myths, so his choice to portray the Oath in much the same light is not surprising. He was even willing to change out participants based on who was politically 'hot' at the time, for maximum effect.