It's rare - maybe five or six times over my sex life of more than 25 years ( started at age 21 ) ; but yes, very occasionally it's so mind blowing that I have cum without being touched by myself or the other guy.
I am sure people think it's to do with the top's size, but it definitely isn't. They were all different sizes. One really small, one really big. A few others either side of average, I'm guessing.
It's also happened with me as top about he same number of times, maybe even a few more, and I'm only marginally over average I think.
In my case, it's about what's going on in my mind. And his. It's like there are "light cones"'or whatever they are called, projecting forward and backward in time and illuminating the possible worlds, as well as defining the barriers. All around me, almost physically present and perceivable, when I'm in a sexually charged state. As if we are different people in different stories. Exchanging words and symbols and meanings ...Sculpting and painting our fantasies into the here and now. When that goes right, there is an almost electric kind of excitement, linking your brain to your gonads and prostate and giving you such a magical kind of surge that you just ejaculate ...
I think it would be too exhausting if it was like that every time. But it's fun to create the right kinds of background, and then go with life's flow ...