i am not the greatest rider or anything but i have been riding for 2 years now and i know quite a bit. It sounded really scary. I havent had something like that but i have had a horse rear above me before. I think he might of gone fast if you tensed up at. To slow him own you needed to relax and show him that you where there and there was no reason to canter madly. Sorry i sound like a boffin but i am just trying to help!!! Good luck if you are going to ride again!!