This is what you would call engine braking, basically the compression of the engine is stopping you because your wheels are out turning your motor even at full throttle. This happens because you only have that one big gear your engine is driving.
The gear multiplies torque in one direction ( engine->gear->tire) but multiplies rpms in the other direction ( tire->gear->engine ) so when going downhill your wheel is spinning at X speed then gets multiplied by first the tires size, then by the size of the gear, and finally by the size of your engine output gear and usually that's more then your engines rpm.
Super technical but given more gears that wouldn't happen.