It's because you are pulling too many of them. They gain a stack for each engaged mob. This mechanic is not unique to this. A monk and lots of dps makes it trivial. Other tanks need to be careful. Crowd control may help. It's not meant to be easy
If you are pulling general trash in expert then pull only up to 5 shrooms as a safe max number. You will see deaths at 6 increments. Ring event shrooms at the first barrier come as following increments: 3, 3, 6, 6, 9 and similar numbers on the second round of ring event before final boss. The 3 stacks should be fine and 6 and 9 will require rotation of saves. To overcome 6/9 stacks, dragoon reflexes work on a warrior tank, they can hold aggro using taunts and let the dps destroy them. If you are running out of saves per round then you can either crowd control by charming last add or slow down dps when you have 2-3 increments and wait for saves to come back up or use a brew of readiness in between. Works either way.
If you are talking the ring events - 8 to 12 mobs in the 3rd to 5th round ? You can also trigger the event and pull back quite a bit, allows time for other party members to do things Then a pet can take the first hit, and or illy/coercer - can mezz some of them. As a Guardian, I use two of "keep me up spells" per the last 3 waves - and it is iffy. In experts - I try not to pull move than 4 to 5 mobs at a time tops.
