On a site called "vetinfo," I found someone in your situation - had gone away, and came back to just one of the cats with diarrhea

: Cats are prone to getting diarrhea when they are stressed. Obviously, I can't say for sure that is the cause, but it isn't really unusual in our practice for cats that are boarded, or even cared for by someone else in their own home, to get diarrhea. This is especially true with boarding, since many cats also have diet changes due to their owners not bringing along their own food. There are a number of possible causes of diarrhea and the only way to sort them out is diagnostic testing. Your vet can help you with this if it becomes necessary.

A general approach to the treatment of diarrhea is to withhold food for 24 to 36 hours and then give a partial meal. If the diarrhea has resolved, it is likely to be due to stress, bacterial overgrowth (occurs with sudden diet changes) or dietary indiscretions, such as too many treats. If not, it may be more serious and it is a good idea to have your vet examine your cat and begin testing to rule out other causes.

Also, look around the house and make sure that there wasn't something the cats got into when your were gone. Boredom can lead to some strange behaviors. We treated a cat that ate a bar of soap when its owner was away and that caused diarrhea.
Any missing soap at your house? Bleaahh!