My question is why the individual who was on the aircraft didn't jump into a seat and apply the brakes?
The brakes use hydraulic pressure from engine-driven hydraulic pumps, with accumulators to maintain park brake pressure when shut down. But these Dash 8s have probably not moved since FlyBe stopped operating in March. Accumulators do not hold pressure indefinitely (which is one good reason why parked aircraft are chocked).
if the accumulator was depressurised, the brakes would not operate until the engines were turning, or the accumulator was recharged with a ground hydraulic rig.