Alex123 wrote:I've read that kiriya cittas are neither kamma nor the result of kamma. If that is so, what is the cause of their arising?
Arahantship, or more specifically, the complete removal of all tendency to think in terms of "I", "me" (i.e. conceit - mana) or "I am" (asmā). Now, if you want to know the cause in a strict Abhidhammic sense, it would be whatever citta(s) correspond to the attainment of arahantship and you could look that up yourself, but that would only answer the "what" and not the "how" or "why".
Alex123 wrote:And in fact, what causes Arahants to behave or think in this or that way? Past (pre-arhatship) kamma?
No, as above. The cause is the cessation of mana.
Sorry for the delayed response.