Williams didn't see who he pushed so it's possible he thought it was a player he was pushing out of the way. He never really looked at the guy.
I know the rule states that when you push a ref it's an ejection, but I think the ref showed solid judgment there in realizing that Williams probably had no idea he was pushing a ref.
I don't think the officials did that bad a job last night. Every crew is going to miss some calls. It happens, even to the best crews.