Help please a Slovakian Jewish prisoner at Auschwitz says:

“When they opened the train carriages and forced us out, they shouted at us immediately. They were screaming in German. They were SS men who were dealing with us. We had to stand in line. Men had to step out first, then women with children, and then old people. I looked at my father, here, and I saw a sad look on his face. This is my last memory of him.”

Why would the SS soldiers begin separating the people arriving in from the trains? What was their reasoning?