maple tree loosing leaves in June
Hello. Happy to help.
I really can't definitively say what it is but it looks like one problem is a gall of some sort; either maple bladder gall or maple spindle gall both are caused by microscopic eriophyid mites. Even though those can cause the thinning of the canopy, I think I see more problems vis-a-vis the discoloration of the leaves which may indicate a fungus. I'm thinking that the tree is stressed. Sometimes the stress can have happened years ago and now you're seeing problems like diseases and insects that seize the opportunity to attack the tree.
I recommend having a certified arborist evaluate your tree. Here is a link to information about how to find and hire a professional: https://extension.umn.edu/yard-and-garden-insects/insect-and-mite-galls#eriophyid-mites-1346961
I hope this information helps.