What is a crackback block? Is the reason it's illegal because it causes injury?

Answer 1

A crackback block is an illegal block by an offensive player who is usually spread out away from the main body of the formation and runs back in towards the ball at the snap, blocking an opponent below the waist or in the back with the force of the block back toward the original position of the ball at the snap.
And yes, it can cause severe injury to the other person's legs.

Answer 2

another crackhead question