It starts with Tony goofing around with Clint, Thor (?), and maybe with the other avengers. I remember Steve seeing that and feeling appalled by the way Tony is acting. I think he felt that Tony should've acted more withdrawn because of his status.
And Steve believed that as an alpha, he had a duty to 'teach' Tony how to be better. So he basically forced Tony to complete this ritual thingy where he had to stay in a position.
In the end, Tony and the other avengers had a talk with Steve and told him that the world's more progressive now and omega's aren't expected to act the way they did in the past, etc, etc.