One of the recurring themes in brain ischemia research is the concept that events that occur during ischemia promote a devastating response during reperfusion.
Think about the last time your foot fell asleep. It
felt a little numb, maybe, but overall not so bad. But when you
relieved the pressure to reperfuse the sleepy foot and it started waking up, all hell broke loose. That's not entirely unlike what happens in brain reperfusion.