I would think cats are like every other creatures and like a bit of a change in their diet now and then, we have found that if we do alter the diet of our cats then it does tend to make their poo runny and oh boy stinky !!! but after a day or so they are usually back to normal, i really dont think changing the diet upsets their tummies, its as you said, it just makes it very runny.. good luck with you're cats.................Chris.