I chalk that up to Anderson's mental state. He was trying to win the title back after having just been KO'd (from acting a fool). In the rematch you could tell he was uncharacteristically timid.
Anderson wasn't going to remain undefeated forever but I absolutely think he wins the first Weidman fight if he takes it seriously. In that case, Weidman doesn't get a rematch and Anderson goes on to defend against other people. Which then we gotta figure out who of those people likely beats him. Probably Romero.