Drawer stop/lock isn't holding a drawer when fully opened. Guessing the clip snapped off.
Just thinking of taking the tops off to review/remedy has me pissed off.
Call ARB they will send you spares no charge. You will continue to break them. I know this because i went through 8 in two years. More weight in the drawers will accelerate this problem.
When I sold my ARB / OS System I included spares and warned the new owner.