I've written extensively on this subject over the years and I am 100% in agreement with you.
Every factor you mentioned plays a role, to a greater or lesser degree depending on the individual, but one factor that most bodybuilders completely neglect is the transverse abdominis. It is almost entirely responsible for maintaining abdominal integrity...and once this muscle is damaged through chronic stretching, the only way to regain that loss of stability/elasticity is to start training the transverse abdominis directly.
Vacuum training, which I have written articles on multiple times, is, I believe, the single best way to fix the problem. However, barely anyone does it anymore, when in reality it should be more important than ever! With all the potential risk factors inherent in modern professional bodybuilding, failing to engage in vacuum training all but guarantees significant waist expansion...and while some of this is unavoidable (especially in those who have gained a lot of size), a large amount of it can be prevented through the implementation of this single training technique.
It's a shame it isn't done more often. I think if more bodybuilders knew just how dramatic of an effect it can have on their waistline, more guys would be doing it.