It's either marking its territory or it hates you:
"Finally, there is the all-out nuclear weapon of insults, reserved only for the most offensive, utterly unacceptable, good for nothing individuals and behavior: urine. And if you've ever had to clean up a pungent puddle of bunny pee, often bright yellow, orange, or even red, and more than a little odorous, then you know just how powerful this weapon really is. Note that urine used for marking territory and ownership is another story entirely. But when your rabbit pees on your pillow (yes, it happens), you have obviously qualified as the lowest of the low."
http://www.paperglyphs.com/rabbits/...ttalk.html