Yes there is! I cant tell you what it is though. I'm currently searching the forums for a way to turn it off. When I go to gmail.com (I have ticked the box to automatically log into my account) it jumps straight to compose. I usually go to gmail to check messages rather than compose them so its a real nuisance.
Didnt see anything in the settings and had'nt knowingly made any changes - perhaps its only a bug but it is certainly possible..