Are you sure you're not just fat fingering and missing the button? You do know that if you tap to the left if the button and miss it you will scroll to the bottom?
I have NEVER has this happen when I press the menu button. I am using Safari on an iPhone 4.
When it happens I make sure I'm not fat fingering when re-producing it (even long-pressed to see the boundary zone for the button and it has still triggered it), happens at random on 5.1 on my 4S and iPad in portrait mode (can't recall it ever triggering in landscape).