The URL ending /qa within your QA site (which in your case is itself at /qa) shows recent questions and answers - it's there in addition to the home page because the home page can optionally be customized to show something different.
If you haven't customized your home page, it shouldn't be linked from anywhere, but obviously Google somehow found it in your case.
I'll check into putting in a redirect in the next 1.4 beta, to prevent this happening.