I looked it up, and figured that when you doubleclick the horizontal scrollbar it zooms all the way out so you can see the whole project and you can't zoom back in just by doubleclicking again. Doing the same thing to the vertical scrollbar makes sense: doubleclick changes the vertical zoom from 1 track, 17 tracks, the whole project and back - that's very good and handy.
Horizontal scrollbar is therefore buggy, definitely. However, it doesn't seem to have a mind of it's own to me.
Cheers!