Wales v Scotland. Usually you foul someone to gain an advantage in a game. But just barging into someone who doesn't have the ball. Why?

I was at the game on Satarday and could here the contact from the top tier, I know that Hogg is better than that and can't imagine what was going through his head. All fans ( wales and Scotland ) agreed after it was a red card but no one had any idea why he commited the foul.