Gotta check the little things, like deraileur pulley bolts, deraileur pivot pins, mounting bolt, cable adjuster, etc. In short, check everything all over the bike. Many times while riders are fretting over the exact sweet spot in their derailleur adjustments for perfect indexing they forget the little things that, when they fail, could be catastrofic.

...or the derailleur just 'gave up' due to fatigue, cheap materials, etc