What went wrong?

What could have possibly caused this to happen? 

The reactor used in Chernobyl was very flawed in it's design. (RBMK-1000)

as well as user error.

The workers in reactor 4 were conducting a test with the plant. 

What made the reactor's design flawed, was how it grew very unstable with low power. And that night in April, the workers were starting the test with the reactor. To ensure it could still work with limited power in blackout conditions. Like if all the power to Soviet Russia went away, this test could ensure the surrounding towns and cities still had power.