I've had the same thing happen occasionally. I initially assumed it was picking up one of my neighbor's codes (never tried the lock-out feature). But this opener had just gotten wet several times because of a leak in the garage, so I was thinking maybe that was causing it to go off on its own. I also noticed that the interior wall-switch that opens the door is cheap and kinda wobbly (it looks like just a door-bell pushbutton actually), so now I'm thinking maybe there's a looseness in the fittings inside the switch; enough vibration might be causing accidental contact.
Or, of course, the opener itself could be bored or possessed...
If you figure out a definitive answer, please let me know.