Rear windows Move a little left and right while open

The problem goes on like this: both rear windows work fine, but what I find strange is that when they are opened, you can move them(tilt; they jiggle) left and right for a little bit. The front ones won't do this thing. So when closed shut they don't move, only while opened. Is this normal with this type of Ford (a friend told that it is)?

Thank for your time.