Probably due to the drought. As long as the cambium layer - the green layer just under the bark, seen if you scratch lightly at the bark of a twig - is green, and the twigs are pliable, I wouldn't worry too much. If you CAN water them, about every 4-6 weeks, very slowly and long, it would probably help, but if you can't, you can't, and they will have to survive on their own.

I have three very old, very large Sycamores on my property. They are constatnly shedding something. Leaves, bark, twigs, furry ball looking things. I hate the trees but love the shade. As Alan Greenspan said, "it is a cunundrum".

~Scott~