It happens when I click the thread title, whether i've been to the thread before or not. It also jumps me to the bottom of every page on multi-page threads.
It's driving me mad.
It doesn't happen when i'm logged out though.
It does it on my laptop and my work computer, bo I guess it's not a mouse settings issue.
I've just rolled-up the quick-reply box and now everything is peachy. Woo hoo