In Chapter 5 of Night, why did the camp evacuate, and what happened to those who stayed behind at the hospital?

In Chapter 5, the camp was to be evacuated because the Russians were approaching.  Those who stayed behind in the hospital "were quite simply liberated by the Russians two days after the evacuation".

Elie had actually been in the hospital recuperating from surgery on his foot when a rumor went around the camp that "the front had suddenly drawn nearer...the Red ARmy...was advancing on Buna, it was only a matter of hours now".  In the chaos that ensued, Elie ran outside to look for his father.  When he found him, the two of them weighed whether their chances of survival would be greater if they stayed at the hospital or if they ran with the others.  After some deliberation, Elie decides that they would be better off being evacuated with the rest of the prisoners.  It seemed unthinkable that the patients at the hospitals would be left unmolested to await their liberators; rumor had it that "all the invalids "would be summarily killed...and sent to the crematory in a final batch".

Elie learned later that had he and his father stayed behind at the hospital, they would have been liberated when the Russians came.  As it is, the two are forced on a grueling march to another camp at Gleiwitz, and then on to Buchenwald (Chapter 5).