You could flip that the other way round to show that BA are proposing the most significant change to the employees that BASSA represent and hence the problem.
If you look back at the original cost saving proposals, the changes were quite modest compared to changes other workgroups have gone through (LHR groundstaff pre T5 move, LGW cabin crew) and the pension fund changes.
Modest changes to crewing levels and a revised disruption agreement are nowhere near as radical as other changes the workforce has gone through. BASSA's refusal to negotiate is the reason why it has dug itself into an ever deeper hole.