I have heard of and personally observed this behavior in both dogs and house rabbits. It may occur among other species as well.
An otherwise perfectly litter trained and/or housebroken pet will get on the bed of person they identify strongly with and urinate. In my experience the behavior is notable in animals that have otherwise perfect house habits.
In my imperfect memory it seems like there is often a relationship to the owner of the bed being away or the animal feeling loss of attention, and this behavior.
Is there a known cause, cure and/or rationale for wetting the masters bed?