laptop randomly comes back from standby

I don’t usually turn my laptop off during the day when I am going between classes. I usually just close the lid so that it goes into standby. However, occasionally now it has been coming back out of standby while it is still in my case–causing it to get extremely hot.

So far there hasn’t been any damage to my machine, but I would like to know any possibilities why my machine is coming back on. I’m running XP media center on it.
I have also tried turning on standby manually before I close the lid. It still happens.