Johnson was an undrafted free agent out of North Alabama. Even though he was the D-II player of the year in 2009, he was probably a long shot to make the roster.
The Chiefs originally signed him right after the draft.
The Chiefs now have 75 players on the roster and can hold 80. As Looney points out, the Chiefs will eventually need some roster room when the seven draft picks officially sign. Also, Jarrad Page hasn't signed his tender yet so he's not official yet either.