If you hold down the mouse button to move the main map around and happen to mouse over the mini-map then it will grab the mini-map and start moving it around instead. This is a bit annoying as you don't re-click you simply are moving around the map and happen to move the cursor over the mini-map.
I'm guesses there is no "on-click" check to begin dragging the mini-map and only a if mouse is pressed then drag. 
