You're Not Using Good Technique
Nothing is more likely to cause injury than using poor form, especially when dealing with the big, compound lifts such as squats, deadlifts and presses. You're number one goal is to always strive for perfect technique, which should be honed through using non-maximal effort lifts. Once your reach technical failure (the ability to complete the rep with perfect form) regardless of the scheduled rep range, end the set. You are better off walking away from a set a few reps short of expectations than not being able to walk away at all.