I don't think you can force it to go one direction, the JavaScript tries to determine which way it can display without going off screen.  This is so if you are near the bottom of the page, the drop down goes up so it can be seen.

My best guess would be that the container the WebCombo is in is getting picked up to be to small on the bottom and its going up instead.  You could submit this to Developer Support, preferably with a sample, so it can be inspected.