Filler up

Birthday present for JJ's Mom. This is her horse Singer who we met this weekend.
We were stuck in MD for an extra day because of the snow; which would have been fine except that the cats got sick, really sick.
Apparently, their food, Petguard, can make them butt sick if they have too much of it at one time. We usually feed the critters three times a day and little bits at a time. While we were gone the cats got fed once a day and the full amount (meaning Fatty ate everything all at once and way more than Petey). Anyway, JJ had the where-with-all to look up info about the food. It can cause butt explosions....why??? It has not fillers or anything else BAD for a cat...what is point of giving them unprocessed food if their butts explode all over the apartment?
Our apartment is clean now and the cats had a very expensive (unnecessary) vet visit. They are now on expensive food provided by the vet to help them be weened back on the natural stuff. And the guess what, the expensive dr. prescribed food has fillers as the 3rd ingredient. Sometimes the irony explodes.