A number of things had to go terribly wrong for Palmer's knee to pop. All of it did. None of it late, negligent, or intentional.
Then why did Von Oehlhoffen wrap his arms around Palmer's leg? Reflex? He didn't hear the referee scream "ball out," which referees do on every pass play and defensive linemen are trained to hear so they can avoid late-hit penalties?
Personally, I don't think the Steelers planned to injure Palmer, and I don't think Von Oehlhoffen was trying to intentionally hurt him. But it was a late hit. Very late. It should have been a penalty.