Skip ahead to 7:48 for Shimano's explanation of Gwin's brake failure.
While initial reports indicated that both of Gwin's front and rear brakes had failed in his final run at the World Championships in Leogang, according to Shimano's engineers in Japan, only the front brake lost power. However, because the front brake will give you the majority of your stopping power, riding the course at race speed with only a functional rear brake was not possible.
From Trek World Racing:
Race favorite Aaron Gwin (USA) saw his hopes of taking the World Title
this year come to an end around 1min 20secs into his final race run in
Leogang, when inexplicably his front brakes failed, resulting in a
crash. Shortly after the crash Aaron pumped the brake to try and get
some pressure going but it was insufficient to complete the course
safely at speed, and he cruised to the line disappointed he was unable
to throw down a full race run. After the brake failure, both brakes were
sent to Shimano Japan and were thoroughly evaluated by their engineers,
and in Norway the results of their findings were revealed to Aaron and
his mechanic Monkey. Testing proto-type products, always to the limit
and sometimes to failure, is part of the role of modern day professional
downhill teams and riders. The timing couldn't have been worse, but the
feedback and information gleaned was extremely valuable and all part of