That is strange. Usually, you have to swipe a bar or button up/down or left/right for the device to be unlocked. Go to ChtEditor >> Lockscreen and try out the three different lockscreens (Manila, Windows, HD Mini) to see if one of them works properly.
Maybe there is a setting for how many pixels the bar or button has to be moved or unlocked and that may be set to zero in your case. Hope someone can point to where that property may be accessed.
Another thing to check would be if this happens if you don't keep the stylus in your device. pm_mazur has reported
here that the device was coming on randomly and was diagnosed to have been caused by a bad stylus sensor. I checked my phone and it does unlock if the stylus is pulled out. If the stylus sensor is bad, it might be getting triggered when the phone is moved and bring it out of the locked status.