Comment by Drupon

Nobody knows exactly why the protester was there. He got up into the tank and talked with the soldiers for a while, then got out and stayed there until someone grabbed him and moved him out of the way.

Given that the tanks were leaving the square, the lack of violence towards the man when he got into the tank, and the public opinion towards the protests at the time was divided (imagine the diversity of opinion on the ICE protests, if protesters had also burned ICE agents alive, hung their corpses up, etc.), it's entirely possible that it was a conservative citizen upset about the unrest who wanted the tanks to stay to maintain order in the square.