Since there was a bit of chatter about the tree in my neighborhood that fell over and compacted the Lexus SUV next door, I thought an update was in order. For starters, it turns out my amateur diagnosis was correct: the monkey grass surrounding the tree is what caused it to rot and then topple over. (It was a pecan tree, I believe, if I’m correctly remembering what my wife told me, which is rare.) We had an arborist come over and inspect a few trees in our yard and the immediate environs. The one in our yard is fine, though it needs more roots showing–a counterintuitive approach in bleach-blond Dallas. A couple across the street, however, are set to come down, mostly due to the same problem that (ahem) befell the car-killing tree.
If you live in a tree-filled area, not a bad idea to get someone out to check on them. The city will do it for free, but it takes a long time. The guy who came to our house was not from the city, did it for free, and was very reasonable on price when it came to fixing a few of the problems. If you want his name, check the comments; I’m sure my wife will jump on with the answer.