It seems managers THINK seniority MAY be equated to age discrimination. The possibility of litigation frightens them, so they try to find another way around the problem. As I understand the situation, seniority is not specified as age discrimination; but time serving can be seen as age related (young people don't appear at the top of the list,) however, rewarding experience is acceptable so seniority may be acceptable for good things, but, possibly, not bad. Notwithstanding all the above neither mangers nor BALPA want to go to court over badly written legislation so the precedent will not be set and both sides will be frightened of the shadows.
BALPA will survive, it doesn't have a seniority system to defend; it has many more issues to contend with that usefully occupy it's time and my money.
Incidentally LIFO is contractual in my company.