#2 Pressure
We're titling this one as pressure because we really can't find any other reason for why he'd do this.
Brock Lesnar was flagged for a violation of USADA's drug policies following his victory over Mark Hunt at UFC 200, and it left a lot of people saying "I told you so". Nevertheless, it was such an avoidable problem as Brock likely could've competed at a high level without the use of performance enhancers, like he had done in the past.
We believe that it was the pressure of the situation that got to Lesnar, and that perhaps there were influences from his camp to boost the likelihood of a victory in Vegas. There was always going to be the risk of this happening, and a lot of pundits predicted that it would be the case long before the fight even took place.
Speaking of which, here's a point that ties into that quite well.
