Can anyone explain to me why a union would do that to it's own members?
1. They know that unless they agree to virtually all BA's requests, BA will walk away.
2. If BA walks away, they have to call a strike, which they know they will lose.
3. They are under enormous political pressure to make this problem go away, given Labour's improved opinion poll position.
4. This is by no means Unite's biggest headache. Consensus forecasts are now for about 500,000 public sector jobs to be lost over the next 5-10 years in order to improve the UK's fiscal deficit. That's a lot of membership fees at risk. None of the "real" Unite officers gives a stuff about 12,000 cabin crew, none of whom are at risk of losing their jobs.
There is always a conflict of interest between the Union as an entity and its members as individuals in disputes of this sort. The Union needs to maximise headcount in order to maximise its own income. Individuals may well prefer to see lower headcount in order to preserve their own earnings. In the BA issue here everyone, other than the Union itself, was offered a winning hand at the start. BA could offer voluntary severance to those who wanted it, and similarly with part time working. Those staff who remained were offered assurances about maintaining current earnings. BA saved the money it needed as a result of the VR and part time moves, coupled with onboard productivity improvements. Only the Union faced losses, from the 1,000 VR-takers who, potentially, would no longer require Union membership.
A long answer to your question, but the short answer is "a conflict of interest" that many people either failed to appreciate or simply ignored.