#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.
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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.