Warning, long comment. Skip forward to the paragraph starting in "Where it becomes specific to Max Planck" if you already understand the psychological roots of toxic work environments in academia.
Some of this is due to the psychology of the scientific mentor-mentee relationship, which has toxic elements nearly everywhere.
Essentially, you have young, highly ambitious people fresh out of college, who dream of achieving big things in science.
They go work for people who have achieved everything they dream of, and who have been successful to a degree only one in thousands of young grad students will ever be. (That's literally the odds if you go work for a Max Planck director.)
The supervisors also happen to have the power to waste many years of the grad student's life - a power only comparable to being able to hand out long prison sentences on a whim.
This alone is a social situation perfectly suited to generate abuse and toxicity. The worst supervisors will cynically take advantage of the situation. The best ones only will have been corrupted by years of bootlicking and pandering into thinking of themselves as the second coming of Christ.
Up to here, this is a structural problem common to all elite research institutions.
Where it becomes specific to Max Planck is in its so-called Harnack principle, a principle that essentially codifies a cult of genius, making it the explicit goal of the society to give nearly limitless financial freedom and executive power to the institute's independent directors and putting the entire organization into these individual's service.
This principle turns that ostensibly modern institution into a time capsule of late 19th century Germany, a Wilhelminian relic. It's poignant and fitting that the society was renamed from Emperor Wilhelm Society after the war.
That this institution specifically is the crown jewels of German science is truly a danger to the standing of German science in the world. Because the society is completely 'democratically' run by its directors, who profit fantastically from the status quo, and due to the near complete lack of accountability and oversight, it is unable to reform.
The moment people speak up against this system, their career is over - making it very easy and convenient for the society to ignore those voices as "anonymous". Of course they are! The fact that we hear about this anyway, every few years, over decades, should tell you all you need to know.