Mountain rotor/wave turbulence's positive side is that usually you're VMC. On the bad side the turbulence is some of the worst you'll find anywhere, certainly at it's worst it will match any thunderstorm, although the IMC factor in thunderstorms makes control loss a higher pobability.
Research on the COS at the time was rotor streams, but all appearances now are a rudder hardover, perhaps coupled with some wave action. I don't know that we'll ever know for sure.