I'm not sure Bayern are playing particularly badly except the suicidally high 3 man backline. I think this is more of a case rock, scissors, paper. Bayern are used to being able to outpace opponents, cranking up the tempo forcing errors. This Salzburg team seems to be even better than Bayern at playing 100 miles an hour.
Their young players are super fast, they are pressing like mad men and don't mind conceding possesion if they can hit Bayern on the break. Not sure they will croak either, they are young.
No clue what Nagelsmann will come up with, Salzburg play like 2012 Dortmund. And Bayern severely miss Thiago in there who could unlock defences. Now it's cut back after cut back without effect into a jammed penalty area.