Most probably all of the above but also the lifestyle they suffer particularly on short haul operations.
They are most probably operating near maximum capability most of their life supplying Air Conditioning and Electrical requirements on the ground.
After engine start they are shut down and then, being located in the unheated part of the airframe, are subjected to very low temperatures for most of the flight. Then, after landing or just prior,they are fired up again. Can't help wondering how the engines on the wings would cope with frequent shut downs and then relight at very low temperatures like minus 40-50 Celsius.
I remember many years back to prove reliability of a particular APU for operation on a certain route it was required to be continuosly operated in flight for a period. No problems. When it was returned to normal operations the usual problems reoccurred.
Seemed an unusual coincidence that when continuous operation was reinstated for a further check..No problems!
But I agree.... they are a damned nuisance, although I suspect the more recent fitments are much better.
[ 28 January 2002: Message edited by: The Vicar ]</p>