Improper Installation. I see alot of guys who tighten stud nuts with crappy impact wrenches and when I take a hand ratchet to them, I can get 3 full turns out of them. Loose nuts will translate into a loose nut to backer to track to head scenerio, which inturn means its sloppy and the whole assembly wont be as strong thus will help the stud to move under acceleration. Not only that, most of the pull throughs Ive seen are from undertightened nuts and holes to close to the outer edges. The real damage IMO comes from understuding, but good installation helps your cause...BBY