Channel change in middle of timer recording

Hi,
I got the Magnavox MDR537H in December, and have had no problems with it until last night. I always record Craig Ferguson, then watch him the next morning before I go to work. This morning, half way through my watching the recording of last night's Ferguson, the channel changed (you could see a "wipe" on the screen) and it started recording a different station.
I have no timer programs that overlap with my Ferguson recording schedule, and have recorded Ferguson every day, problem-free, since December.
My Magnavox unit is hooked directly into the coax first, then via HDMI to the TV.
Does anybody have any idea what could have caused this?

Thanks,
Dave

Yes, I am on cable. Good thought; I had also considered that the cable company rearranged the channels, but I checked, and the channels didn't change.
Some more info, in case it will help - I record Ferguson off of the CBS affiliate, which the QAM tuner picks up. Usually, when I turn the Magnavox on in the morning, it is still on CBS (in my case, channel 8.1) from recording Ferguson the night before. When I was watching my recording from last night, it changed from CBS to NBC (in my case, channel 13.1, so it actually started recording Fallon instead). When I stopped watching the recording, I noticed that the Magnavox was tuned to NBC.