WW has indeed demonstrated a predisposition toward attacking pilots and their working conditions first.
Pilots are a high profile and envied elite in large airlines who make easy targets, not just 'pour encourager les autres' (which certainly helps his aims) but also because there is a clearly defined baseline...he only has to break the resistance and drive crew conditions down toward the legal limits to be seen to succeed. Being an ex-pilot he knows exactly where the weaknesses lie, and how to exploit them - better than with any other group.
Additionally there is a psychological element at play.
Walsh is an ex-pilot, and he had the jesuitical fanaticisim of a convert in the manner he attacked the pilots in Aer Lingus. He not only carried out an illegal lockout against them within weeks of taking the job (the company lost a court case and had to reimburse pilots for that later) but he also used every opportunity given him on the airwaves to describe the profession he once belonged to as being boring and unfulfilling drudgery - surely a contribution above and beyond the call of duty.
It can best be explained as a fervent desire to 'divorce' himself from any vestige or taint of 'the original sin' of membership of that awful profession - piloting. He had to be more Cromwellian than Cromwell himself if he were to satisfy his new buddies in Aer Lingus upper management (not to mention the other staff), that he wasn't going soft on his old buddies in the pilots union.
Who knows, maybe he feels the pilots in BA are indeed the place he has to start picking fights again? After all, he has to prove his credentials to the new audience, and this is a tried and trusted method for success.