Why did my vinaigrette emulsify so quickly?
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.)