Why does the greek word δεσπότης (despótes) in the vocative become δέσποτα (déspota) instead of the normal δεσπότα (despóta)?

Question

In the greek word δεσπότης (despótes), the accent in the vocative case ascends from the penultimate syllabe to the antepenultimate, i.e. δέσποτα (déspota), this being the only exception in words of the first declension.

My question is: is there a known reason for this exception, other than the pronounced effect of the word being greatly enhanced that way; which is also curious, as when the same is attempted in e.g. δημότης (demótes), δεσμώτης (desmṓtes) or εκδότης (ekdótes), the corresponding 'δήμοτα' (démota), 'έκδοτα' (ékdota) or 'δέσμωτα' (désmota) would produce a rather ridiculous effect.
Could it be that the somehow majestically sounding 'δέσποτα' (déspota) is very fitting for the undelying meaning as lord, ruler or king?

Answer 1

It is irregular. A paper “On the accentual status of vocatives in proto-Indo-European” by Dieter Gunkel suggests that the accent on the first syllable in vocatives is an archaism that was retained in words “used to address family members and members of the household” (page 2).