It was never stated why they were reviewing the play, the announcers at the time said they must be reviewing to see if he had stepped out of bounds, but was before they announced what they were reviewing the play for like they do today. If a very vague rule, he waved his arms, but more to tell others to away more than anything else, and he did not put his arms above his head, signaling a fair catch. It could be easily fixed by just passing a rule that states the returner may not signal in anyway and then advance the ball. It sure looked like to me that the Minnesota players did let up, thinking he was fair catching, and then were caught off guard when he started running again.