Cut and pasted from some web site:
GS/Dakar Broken Rear Mudguard
So far there aren't enough cases to call it widespread, but it has happened to a few people. This is the black plastic assembly that attaches to the swing arm and wraps around the rear tire. It's primary purpose, besides being a reportedly legal requirement in Europe, is to prevent mud from splattering up onto the license plate and tail light. It also may prevent some rocks and debris from being thrown up and into the riders following you when off road.
It appears these assemblies fail on their own, or when subjected to the stresses of off road riding. A much higher incidence of failure is reported by those that have installed knobby tires. The assembly is flexible enough that under some circumstances it comes in contact with the rear tire. The result is eventual failure of some part of the assembly. If it goes unnoticed there is a good chance it will be torn partially or completely off.