I've got to go with Allan. It sounds like its going to Standby, and/or Hibernation. If the PC is actually in Standby then pressing any key should get it going again. If it's in Hibernation, then pressing the Power button should start it. The time it takes to get into either state may vary depending on what you have been doing. Apart from general variations, lots of open applications take more time to be saved on disk going into Hibernation.
If the Power button, when held, is set in BIOS to shut down the PC that explains how the PC is finally turns off. Normally you would have to release the Power button and press again to restart the PC. I can't explain why it restarts immediately after powering off. Presumably you checked your Power Options as Allan intimated. Check your BIOS/Power Management settings too.
Edited by Nanobyte, 13 January 2013 - 11:54 PM.