Your theory looks pretty good. I see that the BeQuiet P8 1200W has several clever protective measures which a broken motherboard would probably trigger - like short circuit/under voltage/over voltage etc. etc. all of which turn the PSU off (and probably keep it off for a while).
http://www.bequiet.com/en/powersupply/10
Edit: not sure that link's right, but I think the tech details are the same.
Edit Edit: the link's OK
EDITED: 22 Jul 2015 22:29 by WILLIAMA