One of our older cats won't touch anything with fish in it (except tuna), so we only buy brands without fish byproducts. My guess is that it's milk. Specifically, they're still getting cat milk occasionally. I'm pretty sure that's it. TBH, it isn't quite as bad as I made it out to be. I happened to have a kitten in my lap when I wrote that and, let's just say I was inspired.