IChat in Leopard: Incoming messages launch application?

As a reflex the first thing I do when I open my MacBook pro at home is quit iChat so I don’t get work messages, but on a couple of occasions (like just now), iChat relaunches with an incoming message…...how and why is this? I know I really should know, but I can’t see what setting I need to change and couldn’t find anything in the Help – anyone have some insights on this one?