Cory can swim fast enough to hurt themselves. I have heard of someone who had cory that would propel themselves up into the tank hood, and they would damage themselves, go into shock, and often die(the fact that this had been happening a lot made me think the owner was an idiot however, as he only posted after the fifth time it happened).
This might have happened to you. I suggest releasing cory in the dark, and make sure they have plenty of hiding places.
Be the Better you.