The one thing that hasn't been said, and probably should, is that he was about to become the officials whipping boy.
We see this all the time in the NBA. Early 4th quarter calls going against the team with a big league. Refs know where their bread is buttered, they are in the business of delivering the fans close games down the stretch.
I think Smart has been in that spot so many times before. If you are a ref, and want to whistle a team back into the game, he's an easy player to pick on.
With both of those early 4th calls against him being 50-50, I think Smart recognized two things.
1- he was boiling over emotionally
2- the refs were about to use him to whistle WAS back into the game
And FWIW, he'd be a fool to admit the latter. The way I saw it, taking himself out of the game neutralized the officials, and helped preserve our lead. Very heady move by, Marcus, in my view.