Because you can never have enough beer showers, the Borussia Dortmund boys decided to invade manager Jurgen Klopp's final press conference of the season and dump a few giant glasses of suds over his head as he spoke. They had played their last game, were given their silverware and doomed Frankfurt to relegation, so there really was nothing left to do besides throw some more beer around.
Everyone seemed to enjoy the continuation of the party, though. Everyone except Mr. White Hair sitting on the end. But he's just sad that no one dumped any beer on him.