what would cause new rear sway bar to crack

Hello, it sounds extremely abnormal to have a sway bar crack once, let alone twice. Have the rear suspension checked very closely, using alignment equipment and a physical inspection. In the event something is not properly aligned allowing for too much stress on the bar, a crack could occur under tension. But this is a total "theory" guess so it's really hard to say. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops