What are these two "clips" on the bolts holding by rear disc brake bracket to the frame?
From https://www.jensonusa.com/Shimano-XT-BR-M775-Caliper-Fixing-Bolt:


  Keeps the bolt from backing out. Torq the bolt then slip the lock over the bolt head and rotate in the direction the bolt would unscrew unntil the tabs hit something. This will stop the bolt from turning if it loosens.


Also: https://bicycles.stackexchange.com/a/29154/908