Question

What action was the main cause for Nazis to be charged and tried for at Nuremberg?

Answer 1

participation in the Holocaust

Answer 2

The Holocaust atrocities were the ones that the West focused on. The Soviets were intent on avenging the insult of being attacked. The occupied countries wanted the Nazi Party prosecuted, and the overwhelming majority of the Germans who surrendered to the Soviets never survived the incarceration, or even made it to trial. The charge was "Crimes Against Humanity". so yes @The_Mobster is correct