Why would I black out on Millenium Force?

I suspect it might have been the G-forces (the bottom of roller coaster hills always exert a lot of G-force on the riders), but I don't know why you'd black out this time and not all the other times you've been on it. Maybe you were unusually tired for some reason, or else your brain had taken a beating on one of the other coasters you'd been on, and the Millennium was the last straw.