Economists believe that the most important cause of the increase in income inequality is:

Rapid technological change

Explanation:

Is important to remember that the technology and the science is the most important things in order to a country can progress.

Rapid technological change include a lot of factors such" special technologies like big data, data structures, Internet, machine learning, statistics, artificial intelligence, robotics, 3D printing, biotechnology, nanotechnology, renewable energy technologies, satellite, drone, manufacturing and production technologies".

And is the most important cause of increase income inequality because if we don't have advanced technology the processes are slow and the production is lower compared with a place with advanced technology and proccesses.