Originally Posted by x-voltage
I think it's because he was forced into Slipknot by Mick and Shawn. He never wanted to be in Slipknot, but they forced to beat the shit out of him if he didn't join. It says that on their unmasked dvd. Great story. I think because of this, Corey probably didn't have much screaming technique and that led to his voice being hurt or being changed.
I never saw that DVD but I did know they threatened to kick his ass if he didn't join. But you're probably right about that being why he changed his vocal style.