Given time, Remus might have appreciated that Harry was willing to throw himself back through the fire to avoid the punch.
But, he didn't have that time.
With no target to hit, he over corrected, which on suddenly slippery ground meant he twisted too far and fell. Casting the shield was second nature, instinct born of repetition in training, and that instinct was all that stopped the first of the stunners as he fell below the rest.
Both Baelfire and shield dissolved when he hit and his eyes fluttered closed.