Sounds like something got screwed up and the X fixed it by rebooting. Does it do random reboots very often? If not, I wouldn't worry about it. All computers get hosed from time to time and need to get a fresh boot. If random reboots happen every day or even every couple of days, you've got something going on that's causing a software conflict. Any chance you're running a task killer (like ATK)? Those apps are toxic to the newer OS's we run today.