This happens only in seige battles when the enemy is forced by starvation to sally out and attack you.

It's an exploit to chase routing enemies through the gates of their town. In reality, the defenders would keep the gates closed in that situation rather than save their men by letting them in when enemies are hard on their heels.

You should use a ram to batter the gates down, or sappers to bring down a section of wall, in that situation, just as in any town assault situation.