Thanks everyone! I'm sitting here with a big old DUH! I remember years ago when Allen was a little dude (was Allen ever really little? ) he had some major poop problems... we took him to the vet and she determined that he has a very sensitive tummy, and suggested the Nutro, which is what he's been eating for the past 4 years without issue. Guess which food we gave him when he was younger? Meow Mix.

Yup.... so maybe even that 20% of Meow Mix is enough to throw him and the others into a tailspin. I am going to sift through the food and remove all the Meow Mix (what a fun taks thats gonna be!) And then kill myself afterwards... all that money spent on vet care!