I’ve gotten requests in comments and email for my thoughts, but I really don’t know much more than anyone else. I didn’t see it myself, but this explanation looks like the most likely one to me. Fortunately, it’s also the most benign one. I do think it’s a good reminder, though, that we need to get serious about missile defense.
[Update a couple minutes later]
Here’s a lengthy disquisition on the contrail theory.