At a guess? Given doomposting really took off with the pandemic, I suspect it's a cultural reckoning with untreated mental health issues and a lack of decent coping mechanisms. It's not even just the pandemic, these rates have been rising for a decade or more, I suspect we're going to find out something like a common type of plastic causes mental health issues (among other things - the same diet these days has different effects, which implies it's more than that).