This slap, four days before entering the Champions League in Salzburg, still leaves the "Rekordmeister" with a provisional nine-point lead over Dortmund at the top of the standings.
And Robert Lewandowski at the top of the scoring charts with 26 goals in 22 days.
Borussia can come a little closer on Sunday, provided they beat Union Berlin, which is fighting for a place in the top-4 and qualification for the Champions League.
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This time, no Covid, full infirmary or particular fatigue to explain Bayern's collapse.
The team lined up at the start was made up of all the stars of the club, with the exception of goalkeeper and captain Manuel Neuer, who had knee surgery during the week.
Winner of the last nine Bundesliga, Munich however cultivates a constant: intractable against its direct rivals Dortmund and Leipzig, and in the Champions League, the proud "Rekordmeister" often experiences its worst failures against teams totally within its reach, even poorly ranked, like Mönchengladbach (1-2 in the league and 0-5 in the cup) or Augsburg (1-2).
Should we incriminate the change of system made by Julian Nagelsmann?
Confident in the flexibility and tactical intelligence of his host of internationals, the coach started with a four-man defense and Kimmich as the only sentry, after a few matches in three-man defense with two defensive midfielders.
But Bayern, after having quickly opened the scoring by Lewandowski (1-0, 9th), let themselves be pushed around by a promoted player whom they had nevertheless beaten … 7-0 in the first leg in September.
The Ruhr players pressed high, without fears and without complexes.
Stronger in the duels, surgical in the finish, they were simply the best team.
After equalizing on a penalty (14th), they knocked out the Munich residents in seven minutes at the end of the first period, scoring three goals in quick succession (38th, 40th, 44th).
Bayern, in all their glorious history, had only conceded four or more goals five times in the first half.
The last time in... 1975 against Frankfurt (6-0, 5-0 at the break).
Never before has the team with 31 German championship titles conceded so many goals against a promoted player.
At the break, Nagelsmann released the unfortunate Dayot Upamenaco, completely missed out on his match, and brought in Corentin Tolisso in the middle, to return to the 3-2-4-1 which had worked well lately.
But Bochum, pushed by 10,000 unleashed supporters (health gauge), held on to its feat, and the players in the blue jersey played the half-time of their lives.
Lewandowski only scored once (4-2, 75th), and hit the bar late in the game (85th).
Bochum's players were taught by history: in 1976, the two teams had played a game that became legendary in German football.
After scoring three goals in the first half, Bochum led 4-0 by the 53rd minute.
But Karl-Heinz Rummenigge, Georg Schwarzenbeck, Gerd Müller (doubled) and Uli Hoeness in the 89th minute had reversed the situation and offered Bayern a 6-5 victory in extremis.
In the other matches on Saturday, Wolfsburg and Mönchengladbach, two monuments in danger this season, won each other to move away a little from the red zone.
The “Wolves” won 2-0 in Frankfurt and the “Colts” at home beat Augsburg 3-2.