The cutting disc was a Forney. It has actually been on that grinder for probably 8 years. I don't use the big grinder all that often. It's got almost more power than I can handle.
When the grinder kicked I wasn't really putting any pressure on it, I was just in a weird angle and the grinder was vibrating so bad I put the end of the handle against my knee for extra stability. I think what caused the kick is that my wrist of the hand holding the back of the grinder got a little tired, let the grinder slip down a little bit and put the disc in a slight bind against the sharp edge causing the kick.