Actually, according to representatives on both sides, they could not agree on the language of the liability waiver, and that was the major factor such that the NFL cancelled the event. Apparently, the NFL lawyers had broad language that could have affected Mr. Kaepernick's future legal rights should he be employed by a NFL team.
That's not my understanding.
First, the NFL didn't cancel the workout. Kaepernick announced, about ONE HOUR before the start time, that he had decided to conduct his own workout, at a different location 60 miles away.
Second, my understanding is that the waiver the NFL wanted him to sign had nothing to do with what might happen if he were hired by a team, it was a waiver that made it clear that there was no employer/employee relationship -- and thus no workers compensation claim -- if he were injured during the workout. Kaepernick's side says they returned it with minor edits -- the NFL says they [Kaepernick's people] completely rewrote it, and sent it back at the last minute.
Kaepernick is a clown. An NFL owner would have to be certifiably insane to hire him.