There are probably two possibilities. First, someone else is using a remote on a different DVR and the signals are somehow getting to yours.
Second, you may be describing what seems to be a known, rare and obscure problem with the HR24-500. Mine has similar behavior occasionally (about every six months or so). The DVR scans up and down channel lists, changes channels and selects the guide, all without my doing anything. I can see the light flash on the front of the DVR as if it is receiving a remote command, but the remote is hidden away so it isn't coming from my remote. Some people with amateur radio setups have reported it seems to be some sort of RF interference, but nothing definitive. It only seems to happen with the HR24-500, no other receivers have had this problem reported.
I assume whatever it is it is a hardware problem, maybe associated with the touch panel.