Why does my box unexpectedly change to Preview channel?

Cox San Diego North; Scientific Atlanta Explorer 4240HDC box; no Cox DVR.

I'm watching (or worse, RECORDING) something and the channel unexpectedly switches to COX advertising (Preview) channel.  It does not turn off, then on, just switches.  If I turn the box off, then on, it goes to my selected default station, NOT back to the Preview channel.  So, my settings are fine. I leave the box on all the time, just turn off the TV.