What did the Jews think might be the reason for their deportation?

There were told that they were being 'resettled' in Eastern Europe. According to those people in the extermination camps who sorted the possessions that Jews took with them there were a large number of cooking utensils, and also musical instruments. This suggests that some of the Jews deported to the camps at least thought they might be starting a new life in Eastern Europe.

The Jews thought that they were being sent to work camps, and not to death camps.