What is the Great Resignation?

In the wake of COVID-19 and its precipitous lay-offs and remote working, there is an interesting phenomenon of American laborers quitting their jobs and not re-entering (or being counted as part of) the labor force. 

What is compelling these people to leave their jobs?  What risks are they assuming, or what industry or economic presumptions are they making?

Is this putting pressure on employers?  Was this phenomenon created by employers?