After winning his first seven matches in charge of the club, coach Niko Kovac has now failed to guide his side to victory in three domestic games on the spin – a run that leaves them four points behind early leaders Borussia Dortmund.
Kovac's men looked sluggish from the outset and found themselves two down after just 16 minutes, Alassane Plea and Lars Stindl – making his first appearance of the season – finishing emphatically past Manuel Neuer.
Robert Lewandowski had a goal ruled out for offside in the second period, but any dramatic comeback from Bayern would have been harsh on the visitors, who were deserving winners and added gloss to the scoreline late on through Patrick Herrmann.
The effect of Bayern's recent form on their confidence was evident early on and they were punished after 10 minutes when Plea whipped a wonderful finish into Neuer's bottom-left corner from 20 yards.
The hosts' misery was compounded just six minutes later when Stindl took full advantage of some haphazard defending. Neuer's short pass to Thiago Alcantara was intercepted by Jonas Hofmann, who teed up the Gladbach captain 15 yards out to fire a crisp drive into the bottom-right corner.
A shell-shocked Bayern finally woke from their slumber after the half-hour mark, Arjen Robben and Lewandowski forcing Yann Sommer to make two saves in quick succession, but they were unable to reduce the deficit before the break.
Kovac replaced Robben and Thomas Muller with Franck Ribery and Serge Gnabry at the interval, but they were unable to spark their side into life during a tepid start to the second period.
Bayern were denied a grandstand finish when Lewandowski's effort was ruled out for offside 20 minutes from time, before substitute Herrmann added a third – despite a VAR review – in the 88th minute, drilling past Neuer from eight yards.
What it means: Bayern's start a distant memory
Kovac was in seventh heaven after a blistering start to life with Bayern, but serious questions will now be asked of him after another miserable result. Germany's biggest club find themselves in the unfamiliar position of fifth after seven games – not something their supporters will accept lightly. Monchengladbach, meanwhile, leapfrog their more illustrious opponents in the table and are now level on 14 points with Hertha Berlin and Werder Bremen behind leaders Dortmund.
Stindl sparkles on return to side
He had not played since suffering an ankle injury in April, but Gladbach's captain put in a magnificent display on his return to action. He controlled the midfield against Leon Goretzka and Thiago, and scored his side's second with a lovely turn and finish.
Robben endures woeful landmark game
Robben's 300th appearance for Bayern is not one that will live long in the memory. On a number of occasions the Dutchman took an extra touch instead of shooting and he was duly hooked at half-time by a clearly unimpressed Kovac.
Key Opta Stats
4 - For the first time since 1999 Bayern have failed to win four competitive games in a row during the Oktoberfest. Crisis. #FCBBMG— OptaFranz (@OptaFranz) October 6, 2018
- For the first time since May 2015, Bayern have lost two Bundesliga games in a row. In the first half of the season this had not happened since November 2011.
- Bayern have not scored in 223 minutes in the Bundesliga, their longest drought since 2014.
- Gladbach have now recorded three wins at the Allianz Arena. Only Borussia Dortmund can match that tally as the away team.
Kovac takes his beleaguered side to Wolfsburg after the international break in desperate need of a win, while Monchengladbach will hope to maintain their fine start to the season when they host Mainz at Borussia-Park.