[CentOS] vncviewer and opening mail attachemtns kills the login

I am running centos 5.1
I routinely vnc in from home (centos 5.1 NVIDA laptop) to work centos 
5.1 NVIDIA based station.

works fine 95% of the time.

However, when I am running thunderbird mail client, and there is PDF 
attachment,
clicking on the attachment it prompts to open with evince and I accept - 
my entire sessions is KILLED.
When I reconnect I have been logged out and all my windows etc... are gone.

Is there a reason for this behavior and if so how do I stop it?

Thanks,

Jerry