It could be

a) something that eats memory, when out of memory it reboots. I've seen this.

Or b) there's a watchdog daemon there which reboots the device if it decides that something's not responding. I haven't seen this for a very long time, but in the old times, at least, it sometimes rebooted my N800 due to a problem where the DSP driver didn't respond (that particular DSP problem was fixed with the first OS2007 update) . I guess the watchdog daemon is still there, even though it doesn't see the need to reboot my N800 anymore.

Or c) something I haven't thought about!