The reason is a programming error somewhere, or maybe (very unlikely) a hardware fault.
The meaning is this, very simplified: Every running program owns some memory. If a program tries to use memory it doesn't own, the OS (Linux) will become very angry and kill that program. This is a good thing<tm>.
If I were you, I'd use a better, and OS, client. I suggest Kopete or Gaim. I would also suggest another protocol (Jabber) for much the samme reason, but I realize that the latter may not be as easy.