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)

my issue is if you open the breach and walk away, essentially dont engage with it, because you have the speed boost shrine on it just opens like 2 feet and closes on you?

idk why that is a thing when the breach doesnt does not even open upr