I was referred to a source of curious articles and thought one of them could be of interest:
In summary, the article talks about the dangers of iron and briefly mentions it as being potentially linked to leukemia, though this is not the main focus of the article nor does it specifically mention CML. In any case, you might find the article interesting since it gives some background to the topics being discussed (anemia, iron supplementation, etc).
Nobody ever gave me an answer as to what causes CML, being honest that doesn't matter that much now as it did in the first year of treatment. However, when I come across as something that resembles a possible answer I like taking a look at it.