Why did my vinaigrette congeal?

Did you change the olive oil brand between the successful and thick preparations? The web used to have a lot of articles on how we cannot trust olive oils (even the more costly ones) as they seem more often than not to be diluted with inferior oils. Some have flavorings added. Expert tasters cannot tell the difference, except that your health probably can.

A grocer told me that Chile and California have very strict laws governing olive oils and they inspect them regularly.

I'm wondering if an inferior oil would cause that thickening?

Sorry if this is not the case, but your problem is a puzzlement.

It is unusual for this to happen. One possibility I see is for your mustard to contain a thickener which then also emulsifies the whole sauce. Remember, most emulsifiers will thicken even when present in tiny amounts. If the conditions are right for some other reason (maybe you mixed with an electric appliance or shaked very vigorously) it is not that strange that it can happen.

My suggestion is to try it without the mustard and see if it thickens. If it doesn't, shop around for mustards from thickener-free brands. If it still happens with them, then they worked too well! Normally the mustard is a mediocre emulsifier and allows the vinaigrette to combine, but does not make a stiff emulsion. You could try less mustard then, or adding it at the end and gently stirring with a fork without whipping at all.