I agree with you this season Bayern Munich and Bayer Leverkusen are top teams competing for Bundesliga title. It is sad that Borussia Dortmund is not as strong as they used to be. But Leverkusen has improved greatly under their new coach Xabi Kompany. He has done great job making Leverkusen real challenger to Bayern. This makes league more exciting to watch. Hopefully Dortmund will bounce back and make competition even stronger.
Dortmund's last match against Stuttgart was beyond prediction. I think Dortmund played very poorly at that time. The strategy planned by the Dortmund coach did not go well. The record was finally broken after 4 matches.
It seems to make the reason for the fatigue is not right because these two teams are playing in Europe. Even Stuttgart has to play against Real Madrid. Nuri Sahin seems to have to immediately improve the mentality of Dortmund players if they want to enter the Eurozone for next season. It's only been 4 matches and it's not too late.
I didn't think they would be defeated by Stuttgart but in fact Stuttgart beat Dortmund by quite a large score, this is quite a big defeat for them, with their next match which will face Bochum I think they can win, as long as they can learn from the mistakes that occurred in the previous match which made them lose badly.
https://www.bundesliga.com/en/bundesliga/clubs/borussia-dortmund