Where were the SS going to evacuate the prisoners to? Why did they do that?

The next day there is a roll call, and the head of Buchenwald announces that the camp is to be liquidated. Ten blocks of deportees would be evacuated each day, and no more food would be distributed.

On April 10, the remaining 20,000 prisoners are to be evacuated and the camp blown up. A siren alert occurs, however, and the evacuation is postponed to the next day. Nobody had eaten anything for six days. The next morning the resistance movement suddenly battles the SS in the assembly place. The SS flees, and resistance takes charge of the camp. At six in the evening, the first American tank arrives at Buchenwald.