The first 2009 drop is because universities got conservative in their finances and a lot of job lines, even whole departments/programs, disappeared. At the time this was very apparent — a lot of tenure-track jobs suddenly turned into short-term fellowships, for example.
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There are some who don't like the language of "overproduction" because it implies that tenure-track jobs are the only reason to get a PhD, that it makes the job situation seem toxic, etc.
My sense is that only the people who already have secure jobs (often tenure-track ones!) tend to make that argument and find it compelling, though. I received my PhD in the dark year of 2010, for what it is worth, so that might bias my analysis.