According to PWTorch.com, when Brock Lesnar was previously in WWE (nine-years ago) he did not get along with John Cena. Stories from behind the scenes say that Lesnar was not a fan of a young, up and coming Cena. One source actually stated, "Brock absolutely positively hated and detested John Cena!"
Lesnar reportedly bad-mouthed Cena to Vince McMahon many times, especially anytime Cena was doing something being perceived as positive. It is also said that the feelings "came through" if you watch body language in their first pay-per-view match in 2003 at Backlash.
During that match, there are many moves that look sloppy, and at the time it was believed that it may have been an outright intentional lack of cooperation on Lesnar's part. Some feel that with Cena on the rise, and the animosity that Lesnar felt towards Cena, that it would not have been out of the question for Lesnar to make Cena look green and clumsy. That is a reputation that Cena had at the time, and one that he was working to overcome.