Default [Q] GT-i9100 does not power off

Whenever I press the power button, the phone does not power off, instead it reboots. When I take out the battery and replace it, the phone boots immediately. I've tried a couple of different ROM's: the official CM10.2, an unofficial CM11, my backup of the original JB (4.1.2) and this:
[GUIDE][ICS/JB] 26/12 - Flash Stock firmware/ROM + Root & SIM Unlock[AIO]

It is only a minor annoyance, but still I'd like to know why ?