It probably is not urine- though some women do leak urine or even quite a bit of it when they have an orgasm. It may be ' female ejaculation' although I think that is not a real description of what happens. It is not an ejaculation- a consequence of orgasm that aids the intensity of feeling. Rather, it seems to be some of the same kind of liquid produced by a woman's lubrication. This can collect. for a minority of women, in pockets of some glands and shoots out when orgasm causes contractions.
This can be a lot of liquid or just a little- and other than using a towel, there doesn't seem to be any way to control it or modify the wetness. Its just natural for some women . There is some controversy , I should add, about what the liquid is, and how the mechanics work-- this is just the version that I have come to read in the medical books that seems the most reasonable explanation to me.