It would help to post a screen shot of before and after of what occurs. Either the of the Clips Tab, or the Montage window itself.
Normally, if you move a clip to a new position, the following clip will move so it doesn’t overlap, the clips after it will move by the same amount so the relationship stays the same. It’s not random though.
If you just want to move a single clip without anything else moving, try turning off Ripple Mode in the Edit Ribbon Tab: