The sway bars are just weak from the factory. I can't count the number of these things that I have seen broken. From cars that see weekly track time to everyday drivers that have never even seen a race track. They usually brake right where the sway bar bolts to the lower control arm. Right at the point where the round part of the bar switches to the flat part. Try one of the Steeda rear sway bars. Haven't had a problem with these yet. The factory bars are hollow, the Steeda is solid.