The channel that a timer is set to will switch to the highest virtual channel without notification. It seems to be correlated to whenever a channel changes something about its PSIP settings (channel name, for certain), adds/removes a subchannel, or when there is a power outage. But there may be other reasons too. Sometimes only a single timer is affected, sometimes they all are.
I had a stretch of about 6 weeks without problems, but since then it seems to happen at least once a week, and occasionally multiple times per day.