It's not exactly what has happened.
Machines used to compete on brute strength and endurance. Mankind always used to have advantages in intelligence, communication, sensation, and precision control. All of those are under attack in ways they have not before.
In short, people are rapidly losing a comparative advantage versus machines. The real problem comes when the opportunities for profit from using machines overcomes the opportunities for profit by employing people. The accelerating rates of improvement in technology will make that more and more the case.
Actually, machines used to compete on strength. They required constant maintenance, however; many people were mutilated fixing the machines while they were still running. That was last century.
Then they began competing on precision control; that's been on the rise for the past century.
Communication has been the story of the last thirty years. Sensation, similarly, has been rising for the past twenty years. Intelligence is still in the works.
At no point in this process did machines lead to mass unemployment; indeed, employment has -increased- over the p...
As Multiheaded added, "Personal is Political" stuff like gender relations, etc also may belong here.