My theory is that it began with the discovery/utilization of new oil sources in the US 5-7 years ago. The price of oil crashed by 50% and stayed there for most of the last five years. Many countries around the globe dependent on oil (parts of south america and the middle east) had economic turmoil beginning five years ago, resulting in refugee crises (venezuela, syria, etc.). Refugee activity helped fuel dormant nationalism rhetoric in europe and america, leading to Trump/Brexit, and other movements gaining steam.

Oil discovered -> oil price crashes -> oil countries crash -> refugees flee to us/eu -> nationalism