From the oneworld site, and their announcment of the aer lingus withdrawl - "Aer Lingus' strategy has changed fundamentally since it joined the alliance six years ago and is no longer convergent with oneworld's. The alliance's key target market is the multi-sector, premium, frequent international traveller, while Aer Lingus has repositioned itself as a low fares point-to-point carrier."
It's a cost saving measure. How long before we see Willie Walsh withdraw BA from oneworld as well?