1) I don't think the SD player was running towards Atiki in an aggressive way. They were both getting up and running back up the court and were close to each other. He was coming slightly towards him, but there was no indication the SD player was going to throw a punch or anything like that.
2) Not only did Atiki put his elbow up, but he followed through with his forearm. Didn't look 100% defensive.
I think the ejection was justified. A good lesson to learn to not let your temper get out of hand.