Monday Mystery

Houston is filled with beautiful old trees.
I so appreciate their shade on my daily (sometimes sweaty) walks!

Lately, though, I've noticed many large fallen branches 

. . . and trees covered in brown leaves.  At school, I saw a huge branch loaded onto one of our flat-bed carts.  It was too large for our custodians to carry.  The leaves turned brown and the branches began falling before the cool temperatures, and we haven't had high winds.  What could be causing this?