I still can't understand why the players would have been brought off the pitch.
Because the German FA has a special plan against actions like this.
First step is a short break on the pitch. Second is interruption of the match with the players leaving the pitch for some minutes. Third is that the match totally ends - match would be 0:3 to the opponent!
Some idiot fans in solidarity to some idiot fans of another club... the own club and the other fans do not count.