Possibly squirrel damage? Probably something else, but fox squirrels do make some of our local trees look like that by stripping bark in the spring/early summer. They usually take the bark off the top side of branches at junctions, which eventually kills the distal twigs. The damage may be hard to see from the ground, but easy to tell if this is the problem by lopping off one of the dead branches at live wood.

Here is a link that might be useful: Bark stripping by fox and gray squirrels

No, it doesn't have anything to do with bark stripping. It's just bunches of dead leaves. See picture below.

While I'm no tree expert, Texas A&M Ag Extension has some decent info on pecan trees (being our state tree and all). See link below. It's rather long, but has a ton of info, explaining drought stress and such. Maybe something in there will catch your eye as to what you are experiencing.

Here is a link that might be useful: Texas A&M Agrilife Extension - Pecan Trees

It might be the work of a girdler.

A girdler, eh? That's a really interesting idea. Good clue. Haven't gotten up close enough to see any girdling damage, but the life-cycle and damage profile of these things otherwise fits what I see. There are other pecan trees around in this (residential) neighborhood, and I haven't seen anything like it, but the tree is down by the creek, and all manner of diseases and pests can be washed up by that, I suppose.

There is an elm about 20 yards away, and I don't see any problem with that, as girdlers are supposed to like those as well, but maybe they haven't gotten around to that tree yet.