For several years now there's been a definite nationalist trend in global politics. I've only seen three explanations for it: the Internet, Putin and the global economic crisis which began in 2008. I'll try to examine each one.
The Internet theory starts with the fact that the Net has cut into TV's near-monopoly on shaping public opinion. For most of the TV age it was very difficult to start a TV station. Public discourse was dominated by a small number of powerful corporations and states which owned all the media that mattered. The Internet democratized this system. The revolt of the comment sections, etc.
Humans are tribal by nature. The left-liberal belief system is an unnatural imposition. If it's removed, politics should be expected to become more explicitly tribal.
The biggest argument against this explanation for the current nationalist trend is that the WWW went big in 1995, more than 15 years before politics started to turn in this direction. Why didn't the democratizing effect of the Internet make politics more populist by 2000, 2005 or 2010?
The Putin explanation says that having watched the neocons and their NGO allies foment color revs all around him, having watched them turn public opinion against him in neighboring countries, Putin decided to try his hand at this too, and succeeded.
One argument against this explanation is that the current nationalist trend seems to affect countries in which Putin has no interest. For example it's my impression that Modi is the most Hindu-nationalist prime minister India has ever had. And isn't Abe the most nationalist post-WWII Japanese prime minister? There's Duterte in the Philippines. I think Erdogan is more populist-nationalist than past Turkish leaders, and he's fought on the anti-Putin side in the Syrian war, even shooting down a Russian military jet.
The second argument against the Putin theory has to do with the Ukraine. We're supposed to believe that it's easier for Putin to change public opinion in Wisconsin than in the Kiev Region. If he can make pro-Russian politicians win in America (and seemingly now in France), why can't he do it in the Ukraine, where most of the population is genetically and culturally indistinguishable from Russians?
The biggest argument against the third, economic explanation is that the countries that were most affected by the 2008 crisis don't seem to be more receptive to populism than the world at large. Remember PIIGS? Spain and Portugal have no far right parties of note, Greece has gone in the populist direction and the others are neither here nor there. If economic misery was the reason, I think we'd see more from that uncharitably-named group.
If Putin is eventually succeeded by a Gorby-like "reformer" (Medvedev or someone else), then one of these theories will be seriously tested. If global populism withers after he's gone, we'll know it was him all along. Lacking that I really don't know what the cause is.