It's hinted in the article,

Basically, as this is an average for societies, the average environmental factors are better in Scandinavia than in America. But those factors already peaked, and now are starting a downwards trend:

>The authors note several possible factors, among them worsening health and nutrition, a decline in the quality of education, detrimental changes to media exposure, and the indirect effects of immigration.

OTOH, America's average environmental factors are still improving, so, it's still on the rise:

> It may be that, once we’ve made our schools as good as theirs, we’ll see our Flynn effect max out, and then whatever channels feed their anti-Flynn effect will show up in our data, too.