The 772s and 77Ws are a younger fleet, coming in to AA circa 1996 onwards. Some of the BA 744s were from 1989/1990, so could be 7 years older. The 772s may show a similar issue later in their lives. The 744s have been worked really hard too, which doesn't help reliability later on in their careers. Not sure how hard the AA 777s have been worked, but we'll see 7 years down the line if they have a similar rate of tech issues.

Thing is, that this doesn't explain issues with say CIVX, which is the 3rd newest in the fleet, being delivered September 1998.