Has any other type of sabot done that out of that rifle??

If it happens with more than just this load, I'd say you may have a tight spot or a slight burr somewhere in the rifling. If it's just this particular bullet, I'd advise trying a different brand sabot with those bullets to see if things improve.

I shoot the 300gr SSTs with 80gr of 777 and have not seen that sort of problem out of my Knight. Sometimes the petals ARE peeled back at a full 90deg angle to the base, but not ripped completely off.