I still wonder, I've been with (at least) one sub who was totally out, unresponsive, and limp, from an intense session and that was just my hand. So I wouldn't discount it.

That aside, I've also passed out, in a dissimiliar situation that involved intense emotions and a slight loss of oxygen, (suppressing laughter in the midst of a practical joke I almost fell for while watching others go "all in".) I didn't particularly care for it, waking up slumped on the floor, having literally "fallen off my chair laughing my ass off' , aka FOMCLMAO.

One thing I noted, over the next few years, I could feel myself going out (and managed to stop it) much more easily thereafter.

Perhaps you had a great orgasm (unremembered) and went off eaisly to subspace (you did say you had had an intense session earlier.) I'd explore that more, because anything that sends you their easily is something to be understood.

I've noted the same thing in my girl, who after/during intense sessions, was clearly on an endorphine high and I was able to induce it more and more quickly thereafter. A learned bodily reflex imo.