In the case of Kosovo, there was a relaxation of Serbia's travel rules and a government change in Kosovo itself (a grand coalition of corrution); but there was the snowball effect, too:

Thousands flee economic despair in Kosovo for EU countries, welcome or not - LA Times

But the sudden surge of departures appears to have been motivated by word of mouth. Many would-be migrants say they decided to leave after watching neighbors and relatives depart and having read their Facebook posts about making it to an EU country.

_{*Lunatic*, n.
One whose delusions are out of fashion.}