Why did my vinaigrette emulsify so quickly? When I make salad dressing, I usually don't expect my vinaigrette to emulsify particularly well. I don't do any of the steps described in this question about vinaigrette emulsification, such as drizzling the oil into the acid slowly with much stirring. 

However, last week I decided to try to make a vinaigrette using some aged balsamic vinegar that I just bought. I've used aged balsamic in the past and had the same experience that I normally have with the vinaigrette -- that the mixture doesn't stay together -- I need to stir just before dressing the salad. 

I poured olive oil into my bowl, added my new vinegar, and some salt, pepper, and herbs -- and almost immediately, the mixture turned into the texture of thin mayonnaise. No matter how much oil I added later to thin it down, it didn't break the emulsion. (I ended up saving it as a sauce for chicken, since I don't like thick salad dressing.)

Why did this emulsify so well? Could there have been something about that vinegar? (It's aged and thicker than most, but not so much that it compares unfavorably with other aged balsamic I've bought. It's just vinegar -- no added ingredients.)
You answered your own question here (emphasis mine):

(It's aged and thicker than most, but not so much that it compares unfavorably with other aged balsamic I've bought. It's just vinegar -- no added ingredients.)

Viscosity promotes emulsification by physically slowing down separation. This gives you (and your whisk!) a wider window in which to break the oil into very small droplets. Once established, the viscosity promotes stability. According to Cook's Illustrated, it's due to melanoidins in the vinegar:

These compounds, abundant in aged balsamic vinegar, are formed when sugars and proteins are heated and undergo the Maillard reaction, the chemical reaction that generates deep browning and flavor. Because the molecules of these compounds are extremely large, they increase the viscosity of emulsions so much that it becomes difficult for the oil droplets to move around and coalesce into larger droplets and eventually separate from the water; thus, the dressing is very slow to separate. (Melanoidins also happen to be responsible for the aged vinegar’s inky color.)