That "Bump" is the engine doing exactly what it's designed to do... It's exploding the fuel. But since you're turning it slowly, it's exploding before the piston passes top-dead-center, so instead of kicking the prop forward, it's kicking it back.

Had you given the prop a good flip with a chicken stick, instead of bumping, it would have started (Or at least, it would have ran till the prime burned out)

And welcome to RCU!