It was discussed in the article, but to be more explicit, sometimes a person who is sure of their understanding of things learns a new thing and that new thing opens their eyes to a huge amount of complexity they were missing. They go from feeling like they knew everything there was to know about a thing to feeling like they know little to nothing about the thing. (this is the "Falling off Mount Stupidity")

Depending on how senior they are at work, that can be quite traumatic. A lot of people in tech sort of base their self image on how smart they perceive themselves to be with respect to their peers. When that perception inverts their own world model makes them feel worthless.

In the two cases where people I was managing this occurred (that I knew of) their productivity dropped like a rock and they became seriously depressed. One I managed to get back on track, the other left tech and I have lost track of where they ended up.

Mount Stupidity is the peak of overconfidence greatly outpacing your competency level. So, falling off is essentially being humbled by expreiences that make you realize you do not know as much as you think you do, and your confidence takes a major dive as a result.

Mount Stupidity relates to section two of the blog post where it references a concept related to the Dunning-Kruger effect.