Hi Speedy. Won't claim to be an expert on this, as probably most men aren't. However, what I can tell you is that you're almost definitely not urinating. When me and my partner have sex, she will do exactly the same and our bed ends up soaking wet. Doesn't happen every time and depends upon what we're doing. Is definitely an orgasm, although a different type of orgasm, and happens when there is intense stimulation around her g-spot.
As to stopping it, maybe you should vary things a little and try also to find out yourself what causes it. We know what causes it in my partner, and what doesn't, so think you should really do your own exploration on this.