Borussia Dortmund conceded two goals in three minutes to slump to a 2-1 loss at Freiburg on Saturday and drop 16 points behind leaders Bayern.
VfL Wolfsburg cruised to a 3-0 home win over Freiburg on Sunday to move into third place in the Bundesliga with their third straight win.
Thomas Muller set up one goal and scored a second-half winner as Bayern Munich struggled past Freiburg 2-1 on Sunday.
"We will not start chasing our players and ask them whether their wives are barbers," a Union Berlin spokesperson said.
The German Hairdresser Association has asked the German Football Association (DFB) why footballers have taken to the pitch with fresh haircuts.
Schalke 04 slumped to a 2-0 home defeat by Freiburg on Wednesday, with both goals from Roland Sallai, to extend their winless streak to 28 games.
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