As some maybe aware, Brock Lesnar is going through some "contract issues", and Dave Meltzer discussed the main event changing at the Extreme Rules PPV.
Meltzer reports it is all strictly a storyline, and he noted the idea is to have Roman Reigns take on Lashley in a singles match at Extreme Rules. Reigns will then face Brock Lesnar at SummerSlam, but again this is all subject to change and whatever Vince wants.
“The current storyline is yet another offshoot of the plan to build to an attempt to have Reigns be the babyface when the Lesnar match takes place.
The idea this time is to tell a story that Lesnar is looking at leaving with the belt and going to the UFC”
This whole storyline/angle is to hopefully see the fans feel sympathy towards Roman Reigns. As far as Brock Lesnar’s real-life plans go, Meltzer wrote that Lesnar would need to enter the USADA drug testing pool by June 29th if he wants to fight in the UFC by the end of the year.