Kicked out of Underworld for no reason

Ok so this has been bugging me, I've been doing 600/smite Uw duo and everytime it comes to the "Servants of Grenth" quest my partner and I get kicked out of UW. I'll be 600ing the large group of Terrorweb Dryders and all of a sudden we've been kicked out. Now this isn't my first time doing this quest, been recently its been occuring. Months ago (the last time I 600/smite) this never occured. so if you've got any ideas please share. Thanks

Failing the/any quest somehow?

That's the most obvious question/answer.

Do any of the reapers die? (Lame question maybe, but very possible)

Don't forget that it's not just the reaper that can't die, it's also the Ice King that spawns. I would say there's a decent chance that due to the number of Terrorweb spawns, somebody escaped and wasted the king.

Edit: or even Coldfire Nights and Smite Crawlers could have taken him out if that little area hasn't been cleared.

This has happened with me,when I've been clearing wastes in an UW SC.

Only thing I can think of happening is that a stray group could've hit the Reaper of the Labyrinth.

The usual answer is that some NPC died - it could be the Ice King, or any of the reapers. Most likely, since it happens during that quest, it's the Ice-king or Reaper there that dies, but it could also be co-incidental timing with some other Reaper's death.
And, as Zelgadissan mentioned, it could be smites or coldfires that kill them if you haven't cleared the area.