Why did my vinaigrette congeal?
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.