Usually you just see sunburned fruit on the south side of a tree. This discoloration can be on avocado, lemons, apples and a lot of other larger fruit. I've never seen it on grapes or peaches, but I haven't looked. Often when a canopy thins and exposes fruit that would be in the shade this can be quite common. Most fruit trees will produce fruit in about an 18 inch depth into the canopy from the edge of the canopy. Some fruit trees will produce fruit at the edge of the canopy more reliably, making it easier to pick, but also making it more susceptible to sunburn when conditions arise. Lemons fruit this way.
Recently, some flat sided lemons were brought to my attention
This fruit dries out from underirrigation due to heat exposure on the south and southwest side of the tree. The juice vesicles collapse and cause the flattening. After the fruit sunburns, it often gets infected by a range of different fungal species. Notice the ring of spreading infection in this image
And then the weakened fruit drops on the ground. In this case, it was mainly the flattened fruit that fell before it even got sunburned or infected. These must have been hot trees that didn't the water they needed.
Solution: Keep up with tree water needs.