I'll venture a guess that it's the vinegar. Balsamic vinegar is not a "wine" vinegar. What that means is that the sugar isn't consumed and converted during fermentation of the grape into wine. Balsamic is made by boiling grape pressings to concentrate them. They are then treated the vinegar "mother" and aged. So they have a higher sugar content. Especially "commercial" "Balsamic," the cheaper industrial product. (Not a bad product, since cooking destroys much of what makes expensive traditional Balsamic desirable.) So, it would not be unexpected to get a more rapid caramelization with the higher than usual sugar.
A little less time on the topped side next time, perhaps.
"Kitchen duty is awarded only to those of manifest excellence..." - The Master, Dogen