He says "How can the people not be panic-stricken at the sight of two privileged bodies, and perhaps part of a third, seeming to be disposed in the guise of the Estates-General to determine its future and subject it to a fate as immutable as it would be unhappy?"
I've asked a few questions with varying degrees of success about what division there was among the common people of France, so if anyone has more info on that more general question, too, I'd love to hear it.
The Three Estates of France represent the three parts of the traditional Kingdom of France: The First Estate was that of the Clergy, those that were closest to God and thus First to signify the importance of God to France; the Second Estate was the Nobility, second in importance; and the Third Estate was literally everyone else, which included everyone from extremely wealthy merchants and professionals to the poorest farmer. As you can see, the Estates are formulated along "feudal" lines that are effectively irrelevant by 1789 due to the shifts of economics.
Which leads to the purpose of the convening of the Estates General in 1789, a means to increase revenue to the Crown in order to pay the increasing debt that the wars of Louis XIV, XV, and XVI have put on credit. Due to the movement of economics, the loss of colonial lands, and a rural based economy that doesn't trade well compared to Britain, income in France was falling behind and there were no effective financial institutions (The British had the Bank of England which had been very strong by 1789 to start nearly infinitely funding the wars against France). Taxes were going to be raised, but the issue was where as it was generally accepted that the Third Estate couldn't bear more taxes.
From here, the Estates General needed to decide about how voting would be done, by head or by block. The Third Estate ended up having having six hundred members while the First and Second Estates each had three hundred, thus there was a fear that the Second Estate and the more noble leaning First Estate would be out voted by the liberal First Estate and the Third Estate. Thus the shut out of the members of the Third Estate and the Tennis Court Oath were events caused by this issue of voting.
The tension that Sieyes hints at is the fact that the Third Estate is a lump of people that are not unified by a certain means. The First Estate are bound together by their profession in the Clergy while the Second Estate is bound together by their loyalty to the King and their rank; the Third Estate are varied.
Now, one of many reasons for the French Revolution is due to the disenfranchisement of the bourgeoisie. Before the mid 18th century, there were means of the bourgeoisie to climb into the petite noblesse or lower ranks of the nobility through the purchase of political offices or on the battlefield. These would be slowly closed off by request of the nobility to ensure that the ranks would not be "diluted" and to keep the offices and military ranks for the current nobility. Thus there was a part of the Third Estate that were actively willing to support the "privileged bodies" that would easily work against the "peasantry" of the Third Estate.
Effectively, the political situation of the Estates General was a mess. All three parts of the Estates were fractured by support for a liberal agenda (the First and Second Estates) or a Conservative/Monarchist agenda (The Third Estate) which was different than their perceived voting preference. It was a mess and one of the reasons why there is no singular "cause" for the French Revolution and how it turned out.