The Johnson try is not a typical obstruction as there's no "block runner" or "second man play" involved, he's just dummied to his outside man and broken the line and the outside man has followed through in support.
However, if the support runner in a line break runs in front of the ball carrier and takes out a defender, that has to be an obstruction right? I figured the basis for the call had to be that AFB wasn't clearly in front of Johnson, however I am struggling to convince myself that he wasn't slightly in front when he ran into Mahoney. It would've been very unlucky because he was only momentarily in front of him, but I think he was.