I have had that happen for me as well, I can not always predict when it does but those are some of my best orgasms ever. All I want to do is to do it over and over and over, hehe, one of these days I will find someone that can keep up to me.
The first time it happened to me it really grossed my ex husband out because it left a huge and I do mean huge wet spot on the bed. I didn't even know I had done it and he was not trying to either. It just happened that we had hit on a magic combination of my body position to where he had his fingers. He made me feel pretty embarassed about it. Luckly for me I was with someone after that who thought it was the coolest thing ever.
So ladies if you have it happen for you enjoy it because it is such a awesome feeling and guys make sure if it is the first time to happen for her to tell her how cool you find it. Tell her that any way every single time she does it with you !