I do not recall anyone else ever leaving Valve. I suspect that Kim either has a philisophical disagreement with the company, or more likely, a personal problem. Say, a work relationship that went sour, or a need to move to a new town for other reasons.
Or, it could be that she's simply young, and does not realize how awesome Valve is compared to other companies. If she was one of the original college students who created the prototype for Portal, then Valve was her first workplace. She may not realize how rarely lightning strikes.
Regardless, I'm hopeful that she finds success at her new job.
Dreaming Demon, formerly Tikatt, formerly The Raven