How did problem with Vim, X cause user to be logged off?

[Chris Bennett]

I just wrote a script to check if from a terminal or not, ugly hack, but 
works.

But how can a problem with X result in logging a user off?
I have seen many X crashes from different program problems, but they 
never logged me off before. Sure, X quit, but I was still able to restart X.

What is "special" about this situation? Or is this common and just new 
to me?

Thanks