They started evacuating because they knew that the countries that made up the Allies group were advancing into Nazi-dominated territory.
We can arrive at this answer in the following way:
During World War II, the Allies group was formed by England, USA, Russia, France, and China.
That group wished to fight Germany, consequently, expel them from the dominated territories and finalize the concentration camps.
With the advance of the Allies, the Germans had to retreat and abandon the concentration camps, for fear of being invaded and that the soldiers would be arrested.
As the Nazi concentration camps involved a lot of war crimes, the Germans knew that they would be tried very violently by the Allies and so they decided to evacuate the camps and destroy the evidence that they had been there.
O'Brien approaches Winston and strikes up a conversation. For Winston, this moment is a significant one. Not only does Winston sense that O'Brien's "political orthodoxy was not perfect"
