The pandemic that killed every second European.
Between 1346 and 1353, the Black Death, caused by the bacterium Yersinia pestis, raged through Europe, North Africa and West Asia. Estimates put the death toll at 30 to 60 percent of Europe's population. The consequences were enormous: labour became scarce, wages rose, and serfdom lost its grip in Western Europe.
Useful: To understand why wages or rights change, first look at who has become scarce.
Source: en.wikipedia.org