probably the motor trying to bring it up.
my drivers side went and it was the clips that pull the window up on the cables, they're plastic and quite brittle, one broke on mine so the window was only pulling up from one side, this caused it to try and go up skew wiff (new term) and the anti pinch would come in and send it down.
you need to take the door card out, ther is a screw at the top on the end of the door, and then where your handle is on the inside, the insert in the handle pulls out and there is two screws behind that. these screws hold the door card in place.
once they're undone, ust lift the whole door card up, its hold on hooks, then the door will be free. you will then need to disconnect plugs for your lights and mirrors, and you should be able to see exactly how it works.
Yopu'l be able to check the motor is turning and whether the cables are where they should be. If they're ok, i would imagine its the clips. you'l need to remove the silver frame that is bolted to the outer door and has the motor and glass on it, you will then be able to check the clips fully and replace if necessary. they're on ebay real cheap as mentioned above. I managed to just swap mine round, this fixed it somehow. hope you sort it