According to several sources including PWInsider there are a number of reasons being thrown around why Brock Lesnar has decided to return to the UFC.
The first of course is money and Lesnar’s drawing power as a headliner. Brock is hovering at 40 and rumors say his camp knows his body can’t remain superhuman forever. The suggestion is that years of supplements *ahem* have taken their toll on him and it would require more and more as he gets older. They want to maximize his financial impact while they can.
Second, he doesn’t feel that he can properly train and be WWE champion at the same time. Being that he’s barely there now this seems unlikely.
Rumors also say that he is frustrated with having to be the guy working to get Roman Reigns over with the crowd. He has been vocal in the past about not liking being in anything less than big money matches and feels Roman just isn’t there yet.
The last big talking point is that WWE has some concern that if Brock is in the UFC and loses it could ruin his credibility as WWE champion. This is ridiculous. Lesnar was a legit heavyweight champion and could crush anyone in the locker room on the active WWE roster. Still, if WWE shareholders feel that this is the case then he may be fully released from his WWE contract.
Whatever the case is we will see Brock Lesnar vs heavyweight champion Daniel Cormier soon…assuming he can defeat his toughest opponent – a USADA drug testing cycle.