It's usually the soccer fans who cause main problems during the game - running occasionally on the field adding the job to the security.
This time it got inverted - one of the players decided to throw his t-shirt to the fans as a gift - but then was roughly stopped by policemen guarding the stadium.
Fans got crazy after that and started throwing the plastic seats on the field.
They say that happened because this team was guest playing and won, but policemen were locals and have got behind their team so decided to revenge to the guests and push them hardly. So even people on duty can get involved into the sport battles.