Does anyone know what happened? Fruit still looks good, but she'd like 20% of leaves?

Water stress? sudden weather change? high winds? did the leaves look healthy? pest issues?

Have close to 30 peach trees and never seen such bad a leaf drop at this time of the year. One thing for sure- you need to developed a mulch ring around the tree. Can't grow a healthy peach when grass growing so close to the trunk!

A couple things, it could be captan, it was a bit strong. I think you can only use this product about 3 times. Otherwise not sure what happened? It is a good idea to have a mulch ring, especially on young trees. It doesn't matter as much on older trees. Grass is excellent at nutrient gathering and can steal most of the nutrients. Very effective at out competing trees. Look for borer holes near the base too, any oozing of sap?

Peach trees are usually very heat tolerant. I'm in the same boat as you but my peach tree has fared well with the extreme heat the past few days. Figs are another story!

Did you rule out peach leaf curl?