Thats happening all over the place and has been asked again and again on this forum. I wouldn't worry because it won't help to worry, Just keep it watered regularly to keep the roots alive and wait to see. The last time this happened a few years back, I remember more trees loosing leaves than I am seeing this year here in the park up the street. Most recovered, but some didn't. Some needed some serious pruning. Didn't matter about kind of tree, some made it while others of the same kind didn't. Green limbs are a good sign. If the whole tree becomes brittle, its dead.