The reason why Dortmund lost this match against Mainz is because of the Red card one of the players received in the match if not Mainz can not defeat Dortmund no matter how strong a team is if any player receives a red card their performance will not look good in the match again and they will get defeated, Although Dortmund is not that strong this season and Mainz is also a strong opponent I believe if not because of the red card Dortmund will play very well and Mainz will not win the match.
The red card received in the 27th minute was a disaster for Dortmund. They had to fight with ten players until the end of the match. And it is true that this red card was a strong reason why Dortmund lost this match, because Mainz proved capable of creating goals when Dortmund played with ten players. Luckily Dortmund got a penalty in the 40th minute and resulted in a goal, but still the goal was not able to change the situation and Dortmund had to lose 3-1 to Mainz.
Meanwhile, Bayern Munich managed to win at St. Pauli's headquarters, Jamal Musiala's single goal made Munich steal 3 points from the home team and strengthen their position at the top of the Bundesliga standings, now they are 5 points ahead of Leipzig who are in second place.