"John E" writes:
Probably something in the fruit turns to sugar with time.

I suspect it's starch, which is also a carb. So it's converting an
unflavored carb to a flavored carb (glucose chains breaking down into
individual glucose molecules, for example). I couldn't find (in a
quick search) anything in the USDA database that differentiated
between unripe, ripe, and overripe fruits (except for tomatoes - green
vs red, but no significant difference there).