An airplane doesn't suddenly drop out of the sky because its controls stops responding. So I agree that on the face of it it's either turbulence or some sort of

An airplane doesn't suddenly drop out of the sky because its controls stops responding. So I agree that on the face of it it's either turbulence or some sort of aggressive manoeuvre, either initiated by the pilots accidentally or somehow initiated by the airplanes control systems, not simply "stopped responding to controls".