Breach mechanic

When i am doing a breech, there's loads and loads of monsters popuping up and coming at me. So i kill them while i keep moving away from the hand where it started, following the border of the breach.

At some point all of a sudden there are very few monsters anymore and then shortly after the breach contracts again.

Is this anticlimatic ending normal ? Are there no new monsters anymore because the breach is about to end ? Or does the breach end because i am not killing monsters anymore and are no new ones spawning because maybe there are too many elsewhere that i didn't kill or something ? (which if course i cant get to because its likely not an open map)