I researched this when Cato signed with the Als. Cato has aspirations to go to the NFL. He went to a NFL tryout and was not signed. Immediately afterwards was the start of Riders' minicamp but he declined the offer.
This must have been the time the Riders dropped him from their neg list. Article I read implied that Cato was only interested in NFL tryouts. Whether there were other reasons for not attending such a fatigue or minor injury is unknown.
His rookie agent next contacted all NFL teams without getting anymore invites. What happened next is the agent likely told Cato the reality of the situation, and he reluctantly decided the CFL was his only option right now. By then, the Als had his rights.