Patterson still has options remaining, so he had to clear revocable players (as opposed to irrevocable waivers, which would have been required if he were out of options). Revocable waivers are the type required to trade a player after July 31. Many teams place many players on revocable waivers as a matter of routine, and it isn't very common for another team to claim a player placed on revocable waivers. Perhaps the Cubs also placed some other players on revocable waivers and other teams therefore wouldn't have realized the intent. I don't think placing a player on revocable waivers is public information, so we don't know if Patterson was the only one or not.