There are probably a number of causes. Areas exposed to the afternoon sun, generally on the West, can take a real beating.
The intense heat causes the paint film to expand. If the paint film cannot expand sufficiently, cracks will develop and moisture can get under the paint film and cause even further damage.
On an older home, it is most likely that oil based paint was used at one time. Because oil base paint continues to harden over time, these layers of paint will be extremely brittle. Each layer of paint will expand at different rates which will contribute to the stress on the paint film.
In areas where the old paint has been removed down to bare wood, adhesion is probably much better than those areas with multiple layers of paint. This too can cause problems as the edges of the old paint curl, allowing for moisture penetration.
The most effective long-term solution is to completely remove all old paint. However, with a good job, this will be time consuming, and consequently, an expensive process.