Maybe he did it as a good gesture to Dana and the Fertita's for helping him be on top of the MMA world in record time, giving him a shit load of money and helping when he was on the verge of death.
I can buy this story, it did seem like after the cain fight he wanted no more of it and then out the blue he was in a no1 contender fight with JDS and a coach on TUF 13. I dont know if its true or not, but certainly is a little suspicious. But on the other hand i dont really care, we get to see a better fight and Brock was probably a better personailty to have than Carwin on TUF 13, although i would of prefered it if Mir was the other coach.
If anything, the UFC made more money off Brock Lesnar then he made off them, so they doing him a favor isn't exactly spot on. Everything the UFC does is because it makes money for themselves, they saw the opportunity to sign Brock Lesnar when the HW division was at its weakest, and it paid off.