Looking at the paper here [1], it appears there was stability, then a rise up to 2011 followed by a global drop in 2012/2013, and then more stability (Fig 1). It is implausible that things could change so quickly for everyone. A spurious effect related to methodology is surely the most likely answer? I haven't read the text in detail. Fitting a linear relationship to the above as they do in the paper seems a bit crazy to me, but I'm not a psychologist.
[1] https://www.sciencedirect.com/science/article/pii/S016028962...
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