It depends on how tour groups are handled in Europe. In Asia, some tour groups are not assigned tickets until 24 hours in advance. So while an expected scheduled departure time might be listed as part of the tour, it's not actually a "bump" since there was no ticket issued for the group yet. I don't know if this applies to OP's situation, but this might be a reason for moving 17 people together (if they were never assigned tickets in the first place).