A friend of mine explained it as "We rise to the level of our own incompetence". Start a job at level 1, do a good job, get a promotion/find a new job to rise to level 2. Repeat until suddenly, you were really good at doing X job, but just got promoted/moved to doing Y job and now you are struggling because it's not very related to what you did in the previous position that you were good at.

It makes sense at a certain level, and can explain why we get so many people in positions they don't seem suited for.