His instincts

(which will have been in his genes through
many, many generations) will inform him that he is cornered
and that he should 'fight his way out'. (This is not a good
thing for obvious reasons.) If however you shove him out of
attack range you will trigger his 'flight' response and give
him the instinct to run or freeze because he is no longer a
cornered animal, so there is no longer a reason to fight. He
won't even know this on a conscious level, but thousands of
years of instinct will inform him that he is no longer
cornered and he should run for his very life. Even if he does
not run away, the fact that he feels like running away will
create confusion and self-doubt triggering more adrenaline
and a downward spiral to capitulation.
Once you have created a gap (and the confusion) the
opponent is forced out of attack mode and into escape mode.