A panic email to Craig Bisgeier, who had just done a podcast on layout wiring, was a huge help. He said the likely culprit was that a rail gap or gaps had somehow closed. This can happen in at least two ways: in the summertime when the rails get warm they can sometimes expand, thereby causing intended rail gaps to shrink or disappear altogether. A similar thing happens in the winter when, due to the low humidity, wood will contract and thereby cause the same problem.
This is only an actual problem if you have either a reverse loop on your layout or require rail gaps for other electrical purposes. In my case, I have a reverse loop where I had gapped the rails to separate it from the rest of the layout. And sure enough, those gaps had closed. After a few short minutes with my cut off disc in the Dremel Moto tool and ACC-ing little plastic bits in the gaps to keep the rails from coming together in the future, my problem was solved.
Until this year that is… This time I don't have a problem with gaps closing - I've been having a problem with tracks humping. Apparently, I soldered too many of my railjoints and the rails were too tight everywhere else. So when the inevitable shrinkage of wood occurred, the rails had no place to go - but up. Thankfully, another session with my Dremel cutoff disc solved the problem. I just cut a couple of gaps here and there to relieve the tension on the rails and everything settled back down to normal.
The take away? If you tend to solder your rails together, be sure you're leaving gaps somewhere to relieve any tension on the rails that might occur. I, for one, will never solder turnouts to the adjoining rails again in the future (this also makes it much easier to remove a turnout for maintenance or repair). Also, where you have to have a gap remain open for electrical purposes, be sure to glue little bits of plastic in the gaps to keep the rails from coming together.
Just remember to do what the British Underground riders do and Mind the Gap(s)!