There is a reason your cat pissed on them, it can smell something and wants to mark it's territory.
Either that or it wants to get above you in the pecking order. Buy one of those plug in to the power point Make Cats Feel Good things if it does it anywhere else. I thought it was new age quackery but it actually worked.
As mentioned, it's a territorial thing. It is probably due to stress on the part of your cat, anything can trigger it – a new cat next door, a strange smell in the house, etc.
Creg's example is typical : "Had it happen many times with one of my cats, including one incident where he pissed in a brand new, unworn pair of shoes" The smell of the new shoes probably did it.
Humans don't understand just how important sense of smell is to animals such cats. Apart from all the information they provide, smells can make cats feel secure and insecure. The smell of it's own pee will make a cat feel far more secure. It will mean that it is in it's own territory – in it's own home. Although it can also mark it's territory it in other ways, such as leaving scent by scratching posts/furniture, or by rubbing the side of it's face on objects to leave it's pheromones.
This stuff will help calm your cat down and seriously reduce the urge it might feel to mark it's territory against perceived threats.