Why was the camp evacuated?

The Germans take the decision to evacuate the camp as the Red Army is on its way. The Germans are rapidly losing the war, and the Russians are making substantial gains in the East. It can only be a matter of time before they arrive. Elie is hopeful at the prospect of the Russians' imminent arrival, yet at the same time has every reason to be fearful at what the future may hold. Elie is recovering in the infirmary from surgery on his leg; this places him in a particularly dangerous situation. He's certain that the Germans will kill anyone they deem too sick to make the forthcoming journey, and so he must somehow rouse himself from his sickbed and get moving.
Years after the war, Elie is surprised to discover that the Germans didn't actually kill the infirmary's patients as he'd feared. They were simply left behind and there they remained until the camp was liberated by the Red Army.