In Night, why did all of this happen?

The Holocaust and the cruelty behind it were the reasons that everything in Wiesel's narrative happens.

When Adolf Hitler ascended to German political power, he was able to set in motion his plan for genocide.  This plan became known as the Holocaust, a genocide of Jewish people as well as anyone deemed an enemy of the Third Reich.  The 1944 Nazi entry into Sighet was intended to move the Jewish people there into work and concentration camps.  This marks the start of Wiesel's narrative. Wiesel's entry into Auschwitz- Birkenau and the work camp of Buna, as well as his struggle to survive are because of the Holocaust. He clearly establishes how human cruelty enabled the Holocaust to happen. This is seen in the way Wiesel depicts high-ranking Nazi officials like Dr. Mengele who acted "like a conductor" in the way he orchestrated the murder of millions to the guards like Idek, who took sadistic pleasure in beating prisoners.  Wiesel also shows how this tendency was replicated in the way some victims treated one another.  Moshe the Beadle and Madame Schachter are treated cruelly by fellow victims.  In being able to show the way in which dehumanization is a sadly human trait, Wiesel clearly demonstrates that one of the worst crimes perpetrated in human history was the result of cruelty against one another.  The entire book takes place against this historical and emotional backdrop.