What variable in the operating system determines the fans in OS X?
You might want to download Temperature Monitor (free). It will help determine which part of your system is hot (CPU? GPU?), and when things warm up.

I also recommend you investigate your logs using Console (found in your Applications/Utilities folder) to see if there are any unusual errors which seem to correspond with the temperature spikes.

I would shut down, let it cool off, and observe the logs/temperature after restart to see when the fans kick in again.

Good luck.