When many people react against something in the same way, you can call it a backlash. A backlash against government policies can result in mass protests.
When people get angry enough about something — or just tired of hearing about it — the result can be a backlash. An influx of refugees to a city can cause a backlash against immigration, and the popularity of a particular style of music might eventually cause a backlash against it. A true backlash involves many people acting together. The original, 1815 meaning of backlash was "recoil between parts of a machine." It wasn't until the 1950's that the figurative meaning came into use.