Who knows what the cause is. After I wrote that post, I signed out, went to another computer that had crashed while BR.com was up. Asking it to restore my session (Firefox) got me back to BR.com, signed in.
Since I only access BR.com at one of my home computers, it's no big deal, if perplexing. Hotmail, for example, generally boots me out if my machine crashes.
However, anyone who accesses from a public machine, such as a library or work computer, might want to take extra care about signing out.
borderline useless from hanging around