You could imagine getting down, from normal pedaling to rolling down a hill, being driven by an electric motor, even being catapulted off by a (very) large rubber band.
Every one of these speeds you up. And in every case a force changes your motion.
You could also take a street-level pair of snapshots of your journey:
Only a force acting on you and your bike can speed you up. Just think of trying to get going on ice.
You also need a force to change the direction of the bike, to get you around the corner. Back to imagining riding on ice, if you need convincing.