I had a rabbit who drank an awful lot. I took her to the vet, but he could find nothing wrong. A year later I found that she had kidney stones. By the time I found this out it was too late, and she only had about 2 months to live. I would take your bun to the vet, and specifically ask about kidney stones. I think you need an X-ray to detect them, which is a problem as it is expensive and means anaesthatising bun, but maybe there is something else they can do to check ...