I actually had heard something from a GM about this prior to 3.1 but did not put it together until now.

I would do ZF in one Giant Pull using my DK on a mammoth. What would end up happening is the mobs we come in waves and with totem love he could handle the whole instance coming at him. It was basically every mob accept some of the boss mobs and or event mobs.

A few times I would actually have the ZF instance server, assuming there is such a thing, crash on me when I would have a ton of mobs on my DK and they were all taking AOE damage. All my toons would get disconnected and when I tried to log back in I would get World Server is down. Mal'Ganis itself was still up and I could log into alts but could not log into my ZF toons.

The second time this happened I was getting chracter name already exsits for almost 15 minutes when I put a ticket in. A GM got back to me and the conversation went something like this.

GM: Let me guess you were boosting RAF toons with an 80 and pulling most of ZF.
Me: Yeah pretty much when you don't get dismounted ZF mount pulling ownz.
GM: Yeah this is being addressed in 3.1 create new characters on each account and then delete them it will solve your problem.

That was not word for word and I did'nt really put it together until this post. Apparently other people were crashing the ZF server or at least causing issues from the 100+ mob pulls so they nerfed it in 3.1.