Cats Eliminating Outside of Litterbox?

I have three cats, one twenty year old female, one four year old female, and one year old male. Our family recently discovered that one or more of the cats has been peeing outside the litterbox, and I would like to know if this is due to a disease, or due to the male's aggressional personality. We have two cat boxes. Also, we feed them Meow Mix Hairball control dry food. Is this food high in magnesium or ash (which I found can cause flutd or kidney problems). How do I figure out which cat is responsible, and how do I fix the problem?