This is an example of a legal block by a member of the return team against an opponent who has, or is in the process of, running himself past the play (the direction of the runner) and as a result turns his back toward the blocker who then initiates what is determined to be a legal block. It is not an illegal block in the back due to the action by the defender who “presents his back” to the blocker. No Foul.
This is a play where the runner and the tackler are in a one-on-one struggle which usually means we should be slow and allow the runner a chance to defeat the tackler and advance. In this case though, the tackler “controls” the runner by keeping him in his grasp while driving his straight backwards. After a brief allowance for the runner to break away from the tackler, the official must get on his whistle and blow progress dead at the 26-yardline. Too much control by the tackler for too long to allow the play to continue back at the 21-yardline where he does break loose. Progress stopped at the 26-yardline.
After video review of this play, the call on the “field stands”, completed catch at the 1-yardline BUT the receiver is down inbounds so the clock should continue to run. The only way this could be judged a TD is if the covering official was able to recognize that this play may end up at the goal line and move quickly to the goal line extended. This he/she could determine that the ball was instantly controlled by the receiver and that the ball had broken the plain of the goal line and that his foot was down in the endzone. Only then could the official rule TD. Without being on the goal line, the play stands as called, completed, inbounds, at the 1-yardline.