Personally I think the recent pension-entitlement issue was the tipping point.
Personally you are wrong. Mueller stated from the onset he could turnaround the business which he has, he never stated he could continue that once Aer Lingus was well into lucrative profitability as it currently is. Mueller has set three targets to staff before he leaves, 1) successful resolution to the IASS dispute in Nov/Dec - 2) Final judgment on Ryanair's shareholding in Jan which Mueller intents to allow those shares float into the free market. 3) To ensure continued turnaround of CORE is completed/fulfilled.
That's why there is a move to replace the management team with higher proven track record of people who ensured sustained profitability. There is nothing wrong with that and Aer Lingus' mission now is clear.