Thanks for answer.
So without knowing the GM prep sessions, my running theory is that:
a) The mini-Fountain got corrupted by some Mara-related infestation carried within Berg's hammer, the Fountain tried to cleared it but kinda failed and instead absorbed it in itself, opening the doorway/alerted some Mara to invade Utrix's plane. I guess the Fountain also served in some way as a protecting mechanism against this sort of thing.
b) At the same time Kali released some other, ancient power kept prisoner in that place, which was responsible for killing Zeridos and possibly Utrix (if Utrix can actually die), thereby weakening any remaining defenses of that place.
If that is the case, would be reaaaally awesome to see some clashes between these two forces, wrestling for control of this plane (if they're both interested to stay) and having the players get thrown into the middle. It's always great to see multiple factions, shades of gray, evil vs evil, rather than one big bad kind of thing, imo.
... oh don't mind me, just theorycrafting and thinking out loud. Can't wait for the next show!
P.S.: Agni is awesome, can we keep her please?