1) Contagion. Nice word, right? The surprise victory of Brexit emboldens right-wing nationalist movements elsewhere, and helps their cause across borders. Trump, for example, was openly supportive of Brexit, and the protectionist element of his campaign found common cause with the Brexiters. Trump then wins. Trump invited Le Pen to Trump Tower, and pretty much endorsed her. These events happen in sequence, so perhaps there is a causal link. Perhaps. Evidence? Not much since we haven't seen similar sequences in the past. Democracy, after all, isn't very old, in historical terms, nor is the capacity for cross-national contagion, which requires the very kind of globalism that these hypocritical nitwits abhor.
2) Some kind of international moment, whatever that means. Somethin' in the water, or somethin' like that. Notice how vague I'm being. There is something about global economic or political conditions affecting Euro-American countries helping each of these movements. The problem with this explanation is that the obvious answer just isn't true. The claim of each of these movements is that immigration and global trade are just screwing over the native-born populations of the most economically developed countries. Now, if you know anything about economics, you know that's bullshit and that this is all just racism with veils that range from thin to monomolecular, but that doesn't mean that there isn't something about the perception of immigration, globally at the moment, driving these parties and movements. That's still hard to test. At least survey data can help us here, cross-nationally.
3) Coincidence. A Republican was due to win our general election because a Democrat won the last two. Even so, Trump should have lost because he's a rapist who lusts after his own daughter, and whom voters knew to be incompetent by a 2-1 margin going into the voting booth. Why did they elect him? James Comey, Director of the FBI, violated DoJ policy by intervening in the election two weeks before voting with some bullshit about how maybe he might indict Clinton even though there was no chance that he would. That threw things off, even before we talk about Vladimir Putin. Brexit was very close. Le Pen? She hasn't even won yet. She only made the run-off. Her dad, Adolph, did that once too, and the country freaked out so much that he got crushed, as she probably will too. A fellow neo-Nazi candidate ran in the Netherlands, and everyone said not to count him out because "remember Trump and Brexit!" and he lost, just like the polls said. Maybe this is nothing.
What's going on? We don't know.