#1 The players could be getting complacent
As stated earlier, Bayern has won each of the last six German league titles consecutively, but not only did they win it, but they won it so convincingly that most times, the title was effectively wrapped up with multiple games to spare.
Last season, they won it with a 21 point gap over second-placed Schalke, the seasons before it was 15, 10, 10. This level of comfort in winning the title must have gotten into the players, and created an air of invincibility over them, as despite the portrayal, they are as much human as the rest of us, with the same failings and emotions.
They go into every season expecting to be league champions, and for players such as Muller, Robben and Ribery who have won the title on numerous occasions, the novelty of competition gets lost on them, and they likely partake in the campaign for the sake of it, with the belief firmly rooted in their minds that they would emerge champions, come the end of the season regardless.
This superiority complex creates a false sense of protection, and makes the players reduce the efforts expended into winning games, which in turn reduces their productivity and would see them struggle.
Overconfidence brings about the downfall of all institutions no matter how great it is, and even the slightest form of complacency on the part of the players is bound to take a hit on Bayern's fortunes