The fact that US wine declines more than other wine isn't a theory it's a fact. The reason could have several theories. One is that people are simply boycotting. This is supported by polls etc. But another would be if prices drift apart and a strong dollar makes US wine less attractive relative to (say) French and Italian wines.

As usual the reason overall wine sales would be down e.g. 10% and US wine sales down 20% in a country will be a combination of multiple factors. My pet theory though is that at least for some countries, the boycott factor is the big one.