Air carrier First Officer reported the aircraft rolled uncommanded after chocks were pulled with ground personnel headset still connected to the aircraft. First Officer applied brakes to stop aircraft.

Narrative

Immediately after the 'clear to pull chocks' from the ground crew; the captain released the parking brakes and we started rolling backward. I believe the ground crew said; 'Wait I'm here connected.' As we continued to roll; the Captain and ground crew continued to communicate; and I heard the ground crew saying; 'Wait; the headset's hooked up to the plane.' I made direct eye contact with the Captain and told him we needed to stop several times. I could hear the ground crew saying something like; 'Stop; I'm under here connected to the plane; you're moving too fast.' I told the Captain it's an emergency; we need to stop now. After no response from the Captain and hearing the ground personnel struggle; I quickly applied the brakes to stop the aircraft. The stop was abrupt in order to prevent potential injury to ground crew and to prevent us from entering an active taxiway. Two Flight Attendants had fallen from the quick stop. We returned to the gate to get them cleared by Medical and to have the aircraft checked out for any maintenance irregularities to ensure safety. Both Flight Attendants were cleared by EMS and the airplane was inspected by the Captain with no irregularities detected. No passenger injuries were reported. Preventative measures include: Review and have detailed briefing on airport power-out notes. Follow ground communication procedures and ensure we understand instructions before taking actions.