I recently noticed one of my Post Oak trees started losing its bark towards the top (tree is approx 40 feet tall). In short order, the tree began to die. Right now, the lower 10 feet of the tree is still alive, but I suspect it is not going to make it.

What could have caused this problem?

I live in Burleson also and have 3 dead oaks that the bark is falling off and 1 tree that is still alive but that the bark is falling off the bottom of the tree. I would like to know if this drought related - weaked trees not making it or pest or disease related. If it the later I would like to prevent treat our other oaks before they die. We have Black oaks and another kind of oak that I believe are Post Oak.

I can't tell you what exactly is going wrong with your trees but I can tell you that post oaks and other native trees are very sensitive to changes in their growing conditions. I don't mean changes brought on by Mother Nature as much as the changes we bring.
Trees have been growing for years existing on rain as the primary water source, with many native plants protecting and covering their roots. In too many cases, we have removed the native plants, created a monoculture like turf, and water and fertilize frequently. There are trees growing in similar conditions everywhere in the metroplex; some adapt and some have trouble. I suggest you look at what has been done to these trees that is different than what they have been growing in. Changes in soil depth, watering, digging, concrete placement, compaction, are a few leads.
Tony M