Cats urinate to mark territory.
They do this if they feel the house is too crowded and they have no "personal space" or if you've brought in a new cat recently (or moved into a place that USED to have a cat).
Make sure they have a clean litterbox, and if you feel "something's gotta be done", I suggest a spray bottle with water. When you see that tail go up - squirt 'em.
We did that with our cat...now he lifts his tail and wiggles it, but does not urinate.
