Being in the landscaping business, not a day goes by that I’m not looking at and noticing trees. The only Japanese Maples that have come to my attention lately, seem to have come to my attention because they are holding brown leaves. I’ll be looking to see if I find any that have dropped their leaves.
I suspect it’s likely related to exceptionally high and/or exceptionally low temps and/or drought stress. A few weeks ago, I noticed my brother’s Kousa Dogwood also had leaves that showed drought stress and didn’t color up. I believe they were hanging on, I’ll have to check those too. Next year I’ll be stressing proper watering through the season, to protect against another year of stress that would definitely increase the danger to the tree.