B737-800 flight crew reported rejecting the takeoff after experiencing uncommanded yaw during takeoff roll.

Narrative

At approximately 120 kts on takeoff roll we experienced an aggressive uncommanded yaw to the right. I felt the airplane may be unsafe to fly so I made a decision to abort. We stopped the airplane safely on the runway; had all passengers and FAs remain seated. We taxied off the runway and determined it would be safe to return to the gate.During this event we had no warnings on flight deck; only the very sudden yaw. As we taxied off the runway I did notice a master caution flicker and then I saw an aft service door light come on. After I talked to the FAs in the aft I discovered that one of the FAs came out of her seat during the abort and accidentally hit the aft service door handle. She subsequently stowed the door handle and the warning light went out. She claims she was buckled into the seat properly. She reported she was not injured.There were no injuries to passengers or crew members. We taxied back to the gate uneventfully and reported to maintenance.Cause: I have no idea what caused the uncommanded yaw. I hope maintenance discovers the cause. I would like to know.