My horse used to do this while I was picking out one particular hoof.. If I started with that hoof he would literally fall over backwards. Still haven't figured out why but he doesn't do it anymore.
We also had one that did the same as your boy, she would start shaking like crazy before she lost her legs though
only if you scratched her in just the right spot though
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms