Door shaking on the highway??

My E has been doing this for a while. My driver's side door literally shakes in and out on the top, it looks like somebody is trying to yank the door open from the outside while driving on the interstate.

Why would it do this?? It doesn't cause rattling or anything and I can't feel wind leaking in. It's just really damn annoying because I can see it out if the corner of my eye and obviously if the door is shaking the mirror does too.

The door is perfectly flush on the outside. The only thing I know is that the weatherstripping tore when I messed with it, causing awful wind noise. However this shake has happened since way before that, and I have no idea why.