Hey--I second the ideas about 1) urinary issues or 2) territoriality.

Mosie pees in the bathtub whenever he has a urinary tract infection. I'm sorry your kitty picked your bed!!

If he's not acting unhappy about his pee-function (grooming his pee-pee a lot, for instance), I'd bet territoriality ... male cats have a tough time with each other, esp. when one isn't neutered (which I don't know for sure, am just inferring from another comment).

:( Let me know how it goes.