Ha! It was never a red, barely a yellow, only because it "looked bad" and Kerry made such a hoo-haa about it. Watch it in very slow-mo, Dec OS starts to go downwards just as he runs into Brennan, trying to stop I suppose, slips a bit. Brennan naturally has his arms up to brace for the collision.
On what planet do you think Brennan wanted to raise his arms to smash O'Sullivan in the face? He even closes his eyes in anticiaption of O'Sullivan running into him FFS! Maybe you think he should have just stood there and waited for Deccy to floor him with no attempt to protect himself?
Take a moment to digestive all of that.