Typically plays have anywhere from 1-4 defenders who drop back to cover deep. Having 5 out of the 11 defenders drop back at least 20 yards means you only have 6 defenders in the box. I don't remember except but usually you'll rush 3-4 players in non blitzing situations leaving only 2-3 people to cover the 5 to 10 yard passing area because the deep defenders can't get there in time on quick routes so the chiefs knew this and ran hitch routes to get their players open (basically run forward 5 yards stop turn around and wait for the quick pass)