What is causing the branches on my pecan tree to turn brown and die?
I would suggest removing those branches and watering the tree well. It could be that an animal (squirrel?) damaged the branches. Or it could be that they're dry and brittle and the bark is simply flaking off of more-stressed limbs. Or it could be a combination of the two. Pecan tree limbs are prone to becoming weak and brittle in drought stress.
In any event, the damage is physical and does not appear to be insect-related or pathogenic. Be sure to water the tree deeply and thoroughly, at the dripline and beyond. You should also consider fertilizing the tree. Here are some links to more Extension information on pecan trees.