To answer your question "Why did the second police officers act in this way? Did he have instructions?"
You don't break the skirmish line. Hold the line, push forward. hold the line.
Following the skirmish lines are other personnel that are not involved with "holding the line".
The cop wanted to help. The cop behind him reminded him of procedure and training, He probably said something like "hold the line, support will be on this. Hold the line."
Having done plenty of Civil Disobedience Training myself in my nasty guard time, that's what I thought.