Brock Lesnar's desire to face UFC light heavyweight champion Jon Jones has not only put the SummerSlam booking in limbo, but WrestleMania as well. While Lesnar will definitely be competing at both events if physically able, there is a lot of maneuvering going on behind the scenes between WWE, Lesnar and the UFC.
WWE's preference is for the Lesnar vs. Jones showdown to take place as soon as possible, with WWE being willing to allow the fight if they're cut in on a small percentage like they were for Lesnar's previous UFC contest. However, Lesnar and the UFC prefer for the fight to take place after Lesnar's WWE contract expires.
Vince McMahon wants the fight to happen before the Royal Rumble, right when Lesnar's athletic commission suspension expires. Win or lose, as long as Lesnar isn't the Universal Champion, McMahon feels the fight benefits WWE as it would raise Lesnar's profile heading into WrestleMania. Also, with the fight out of the way, McMahon believes it increases the chances of Lesnar signing a contract extension with WWE after WrestleMania.
On the other hand, Lesnar and UFC both prefer for the fight to take place after WrestleMania, when Lesnar would be a free agent and the fight wouldn't need WWE's blessing, nor would WWE be entitled to a percentage of the promotion. UFC is also hopeful of signing Lesnar to a multifight deal, rather than signing him for just one fight and having to go through WWE to do so.
But until all of this is settled, it's difficult for WWE to figure out what to do about the Universal Championship, not just at SummerSlam, but at WrestleMania as well. When they shot the angle to explain a potential Lesnar absence in the event that he lost the Universal Title at SummerSlam, the expectation was that Lesnar would go on to face Jones in late December or mid-January. Lesnar's preference to not rush the fight has WWE officials concerned that he plans to leave after WrestleMania.