OK, I will try and explain my opinion (and not that of BA, Unite or anyone else) on why Unite members may vote in favour of industrial action due to imposition.
If BA chose to retire the B744 fleet and did not replace it, then you would be left with an over established crew community. This would probably mean a standard redundancy situation.
This is not the situation within the crew community now. There was a change to the on board "agreed" establishment figures. This was imposed. The effective headcount was reduced by 1000 and it was voluntary. However, there are still 12000 crew left, who will be working extra on each flight to cover the reduction. Previously, there was a payment to compensate for that, but not post imposition.
Now, there will be a ballot result tomorrow to see if the cabin crew community wish to take industrial action because of this.