I don't claim to be an epidemiological expert, but one possible cause the author doesn't really grapple with is the serious decline in "fatherland prestige," for lack of a better term, that occurred when the Soviet Union imploded. Russians went from being citizens of one of the world's two superpowers to being citizens of a much diminished state.
The remnant Russia had shed many of its SSR components (e.g., Latvia, Moldova, Ukraine, Kazakhstan, Belarus, etc.). It also lost its role as leader of both the Warsaw Pact and of a worldwide Communist movement. That is a lot of prestige to forfeit at one time. It could cause widespread depression.
I wonder if something like this didn't happen to France when Napoleon was defeated, to Germany when Hitler was defeated, to the U.K. when they gave away the empire? Perhaps the data weren't collected at the time to enable us to answer those questions.