I got this weird bug recently: When mouse-hovering the dash panel, the activities overview shows up. Why is that? (I'm using "dash to panel" extension).
I'm using Ubuntu 17.10
Activities getting activated by mouse hover at the top-left corner is in fact a default GNOME shell behaviour. But in Ubuntu 17.10 it's patched not to do so.
I reckon by installing Dash to Panel (or some other extension(s)/package(s)) you have broken the patch somehow.