PowerMac G4 won't shut down

I have a PowerMac 64 Digital Audio with a Sonnet 1.4 Ghz upgrade card in it. After reading a few opinions, I've been keeping it on all the time. Last night, I tried to shut it down for the evening... just to give it some down time, and it shut down OK, but then restarted 5 seconds later. I thought I had just hit restart by accident, but then proceeded to shut it down properly, and have it start up again without me doing anything. Does anyone know why this might be happening? Could the shutdown and restart scripts have switched? Is it trying to do something at a certain time and needed to be on? Thanks for any help.