I'm probably one of the least qualified people to answer anything, but here goes.
Pasty butt is when a chick has a piece of poop that dries blocking the vent. Some chicks may just have some dried under the vent. If left untreated the chick can no longer defecate and could die. A warm, wet washcloth or paper towel can be enough to wipe it away. Otherwise, soak the chicks bottom in some warm water for a couple minutes, then wipe away the mess.
Lightly blow dry their bottoms so they don't get chilled. I just had to do this this morning, which cracked up my husband.
We've had our chicks since Monday and I've had to clean two of them so far.
As far as causes or prevention, I'll leave that to the pros.