I've had that happen 2-3 times during tough strength workouts - usually things involving jumping and/or up-and-down action. Burpees involve both. :( Like you, I'm not sure what brings it on when it happens, given that I'm usually fine. I've wondered about hydration, calories or fatigue from earlier workouts.
thinking it through..pretty sure dehydration.
Part of me is jealous of your high intensity workouts, which I really shouldn’t do for health reasons. Another part is glad I don’t put myself through that now- sounds brutal.
Most mortals might feel like blacking out at that level of effort.