There was a bug report about unending streams of enemies in Dreadfell before, which I can't find now, but I think this could be a potential explanation. On level 2 of Dreadfell I suddenly encountered wave upon wave of enemy. I then noticed they all seemed to be around level 47, indicating they were vault monsters, and yet I hadn't opened any vaults on the level. Eventually I fought through them and found this:
Vault door is at the bottom (unopened), but there was a gap at the side which they leaked through. There were no wall-digging monsters in the vault that I noticed.
On a lesser character this might have been a game-ender, as huge waves of level 47 nasties in an open area ain't so nice.