Probably in a drying wind. Japanese maples require shelter from wind and will take full sun if in such a sheltered spot where the wind doesn't whistle over them. The roots and stem are simply unable to keep up with the transpiration (passage of water out through the leaves), and as a result the leaves start to shrivel. Even extra water won't help if they're in a windy position

Agree with Worthy's answer, except the dissectum varieties really don't like very hot sun in the middle of the day, that also frazzles the leaves, so likely the heatwave caused a problem. We haven't had a heatwave for some years....

If its in a pot, place it in the shade, if its in the ground in an exposed spot then think about moving it, not sure when would be the right time, I'm sure someone on here would be able to tell you.