An Aboleth, dwelling in the murky subterranean tunnels beneath the swamp, is dominating the druids. (Or the druids consider it such a great threat that they're prioritizing dealing with the Aboleth and its minions.)

The fey are messing around with the druids, and a Gray Jester (Heroes of Horror) is turning the druids into Bleak Ones, which are essentially non-undead zombies.

The radical druid who's had enough of civilization also works, and if it's a high-level enough campaign he could make use of the Scouring Constructs in Monster Manual V. Druids building constructs?! CR 18 ones, at that.