EMB-175 First Officer reported after the jetbridge disconnected the aircraft rolled backwards due to the brakes not being set.

Narrative

Captain and I were entering takeoff data when the jetbridge started backing up. I felt the sensation we were rolling backwards--like I often do when the jetbridge leaves the airplane. After a second the sensation we were moving backwards continued and so I looked up to find we actually were moving backwards. A ramp personnel was signaling to set brakes and I told the captain to set the brakes. At first; he just used the foot brakes; which did not stop the plane and then he used the parking/emergency brake which stopped the plane after we rolled back a little. We then got on comms with ramp and determined that the were all good down there and there was no damage to aircraft or tow. With that we proceeded to depart normally.Cause: I'm not 100% why the brakes were released but I think the captain dropped the brake in order to ensure an on-time departure and then may have mistakenly forgot to reset it. I don't know why the brakes on the tow (because it was connected) or other mechanisms didn't stop the airplane. But I do believe this was at least partly caused by our mistake for having the parking brake off without ramp knowing.Suggestions: I'm not familiar with the ramp personnel's processes and procedures but something may have gone wrong there to allow this to happen. In our case; we should make sure the ramp is ready before parking brake is released.