We can't determine the exact cause of the twig dropping, though squirrels clipping-off the branch tips with their teeth is a possibility as they need to gnaw on things to keep their ever-growing teeth worn-down. Some beetle species sever twigs as the adults or larvae chew into them, but they tend not to cause this type of damage this time of year, nor this extensively or in that style (cut at an angle). Perhaps these are weakened stems that acquired periodical cicada egg-laying damage a couple of years ago, and as other issues have affected the tree they are becoming more brittle and falling off. If you find the scarring pictured in the linked page on those stems, it doesn't necessarily mean it's the sole reason for them falling (as most tree twigs received that kind of damage that year), but it's something to consider.

The issues plaguing the tree overall are hard to diagnose given the limited details we can see in the photo, but unfortunately based on how extensive the leaf desiccation is, we suspect the tree is dead or too weak to recover. Still, to help make a diagnosis to avoid the problem from recurring with other trees or a replacement for this one, it may be useful to seek the opinion of a certified arborist who can assess the tree in person. Some are employed by tree-care companies but others consult independently in case you are concerned about a bias towards services offered by that company.

Miri