I didn't see the play where he was injured, but I have always thought that the 'boards' (for lack of a better term) were a bad idea (from what I understand, Arena Football is played in hockey arenas in most cities - the barrier that keeps the puck on the ice I would think could lead to spine compression injuries when someone goes head-first into the padding/fiberglass).
He was injured in the middle of the field during a normal (for football) play. The Guard came at him, then the FB and then the ball carrier.
In the AFL the boards are made out of 10-15 inch foam before the actual boards.
This is just a freak accident which could have happened just as easily in the NFL.