He's doing it on purpose.

First thought was, photomanipulated? The triangle of the dress being stretched looked darker, possibly faked. But I found another photo of the moment. so I guess it's legit.

Second thought, "so self-absorbed, even when looking down, he doesn't notice he is stepping on his wife's dress." You're making adult assumptions about the kid. Think teasing grade-school mentality, and you realize he's doing it on purpose! ;)