Google "exercise hypotension". This is different than postural hypotension.
You want to get a good feel for it, go do a set of 30 heavy squats. You'll have to hold onto the bar when you're done. I can't remember the exact mechanism, but heavy exercise causes changes to the chemistry that regulates blood pressure. When you suddenly stop exercising, it takes a few seconds (that's seconds) for things to get re-regulated. During that time, you feel pretty crappy and may nearly pass out.
But you've watched TdF on TV, right? When those sprinters hit the line, they just stop after a hundred feet or less, right? No cool down, or no more than a few seconds of cool down. I never cool down, because the last bit to the car or house will always be taken a bit more slowly, and it only takes a few seconds for things to adjust.
People who experience the worst post exercise hypotension usually do so as a result of heart disease. I've never known anyone to drop dead at the end of a ride, though I have known of club riders who did so in the middle of a climb. In fact, I've never "heard of the guy."