What's the meaning of the error?

I run a program on two GTX295 (4 GPU) in linux(centos 5.3).

But in the middle of the running the program was stopped, several seconds later my account was logout automatically and come into the login interface.

Who know why? The log (in /var/log/messages ) is following:

Not sure what that error means, but this is why you were “logged out”:

Xorg crashed, and you were dumped back out to gdm (which restarts Xorg when it terminates).