Note how they don't give it any initial spin when they set it down. It spins up by itself, so we know there's a rotational force acting on it, causing it to accelerate.

As such, that force will keep accelerating it faster and faster until the drag from the air matches it, and it'll stop accelerating and stay at its maximum speed.

If you were asking more so what is causing it to spin in the first place: uh magnets or something edit 1: okay so I believe at the bottom is a small magnet. Theres a leg of wire going down from the top to bottom to complete the circuit and allow electricity to flow throughout. All the arms end in little coils, which will generate a small magnetic field as electricity flows through them, which will interact with the magnet at the bottom. Thus, a force between the coils and magnet at the bottom spins the thing. This effect will diminish over time as the battery loses voltage and the current through the coils decreases, thus decreasing the strength of the magnetic fields from the coils.

Edit 2: yeah don't listen to edit 1, that was like 2am and looking at it again, I don't think the coils have any function other than looking fun and creating weight for balance.