What causes pasty butt?

[Tala]

For mine it was stress I think. The ones being handled at the feed store came home with runny poo that stuck a little (although no one ended up pasting up! yay!).

 

[sara]

I've never had an issue with this ever until now. However with my most recent hatch I have some. It's weird because it's only on one breed from one breeder. All the rest that hatched that day are fine. I'm a bit puzzled.