Why/when would grapes (and other fruits) have better flavor/texture when served chilled?
It's because different taste buds are activated differently depending on temperature.

A new study reveals why our taste perception is enhanced as the temperature of food and beverage products increases, explaining why beer is more bitter and ice cream is sweeter when consumed warm.
......The study... identified microscopic channels in our tste buds - termed TRPM5 - as being responsible for different taste perception at different temperatures.
According to our researchers, the reaction of TRPM5 in our taste buds is much more intense when the temperature of food or fluid in increased, sending a stronger electrical signal tot he brain and resulting in enhanced taste.

Note, this particular article is over 10 years old.  There has been a lot of research that has both confirmed and extrapolated greater details on this.
Food temperature affects taste, reveal scientists - Food Navigator
Other, more recent articles -
The Guardian (2013): Hot or not? How serving temperature affects the way food tastes
Flavors Fluctuate with Temperature - Scientific American (2012)
In the case of your grapes, there is probably some kind of flavor profile or combination that is greatly enhanced by temperature. If the skins have a pungent bitterness that is enhanced at a greater temperature, it might overpower the flavors you like, while at a lower temperature, it gives a hint of those flavors that is more pleasing, while the grape also has enough sweetness that still carries through while cold.
That is probably a much more likely scenario than the particular food rapidly ripening or decomposing to that degree.