In this case a number of them are whipped into an anti-Ukrainian frenzy by a bunch of rumors. When the Westerners are called "Banderist" it calls to mind a small number of things--actively fighting Stalin, helping Hitler, and ethnically cleansing Poles.
Repealing a language rights law was deemed the same as making Russian illegal. They think that's coming. They fear repression--they still rather like Stalin and Khrushchev in some areas and view their actions as reasonable. But they fear such "reasonable" actions being imposed on them.
They also are terrified. The Donbas is not prosperous. It's obsolete heavy industry. High unemployment, high social dysfunction. Not all that educated. If the political power shifts west for any length of time, they'll be even less prosperous.
They're already pre-demogogued. They tend to vote as a nationalist bloc. They tend to think of themselves as both privileged and underserved, but the most important part of Ukraine. They're ripe for populist thinking, through no fault of their own.
There's also the issue of the Berkut, who have every interest in revenge. Their best course for regaining prestige and importance after being slighted by the ethnic Ukrainians and humiliated by their loss and the exit of their boss is to get Russia to take over and restore their role. If they fail in that, this kind of activity at least restores their sense of personal integrity, as they get revenge for what they perceive as an unmerited wrong.