BA have got a pilot surplus. Not a big one, but certainly on SH Airbus they've over-cooked recruitment. Some say they did this because late 2017 was the provisional delivery date for the the A350. But someone with a beard and a tabard cancelled that plan at what seems, very short notice. So BA have now got too many A320 pilots at LHR. So they're allowing some to leave during engagement freezes (first 5 years) as FPP's arrive.
Hence the slowdown.
Watching the vultures on the carcass here, I'd like to add : we're good. Find us a new CEO and we are better. But your armchair hypotheses are to be honest, a load of b......