I've had this happen once. I was very worked up in the first place and he came in my mouth. The taste of him put me over the edge. His reaction to this was funny, he was very surprised and had this so very male expression of like 'Yeah, i'm good.' hahaha
More common is I can easily achieve orgasm through just playing with my boobs and nipples, especially if there is a sexy someone sucking and biting them
If youre asing why does this happen physically its likely the result of a buildup of the hormone of oxytocin which is released when a female is sexually aroused.