This question is regarding the phenomenon of spontaneous nasalisation (emergence of nasalisation out of nowhere) in New-Indo-Aryan as it evolved from Middle-Indo-Aryan. This is a well-documented phenomenon: MIA assu 'teardrop' > Hindustani ā̃sū 'teardrop'; MIA akkhi 'eye' > Hindustani ā̃kh 'eye'; MIA niddā 'sleep' > Hindustani nīnd 'sleep'; MIA has- 'laugh' > Hindustani hãs- 'laugh'; and many other such examples. This is found in other Indo-Aryan languages as well. Many of these instances of spontaneous nasalisation occur in words which have undergone the extremely common phenomenon of geminate consonants simplifying to single, and the preceding vowel lengthening due to compensatory lengthening (as in the first three examples that I've provided here). My question is, why does the vowel undergo nasalisation as well? Is there some articulatory explanation that explains why the nasalisation seemingly emerges out of nowhere? I ask this question because as far as I know, there is no research on the articulatory aspects of spontaneous nasalisation, or at least I haven't been able to find any study on it.
It seems to me that you're answering your own question. The sound change is not "spontaneous". What you describe is a chain: geminated aCCa > prenasalized anCa > nasalized long vowel ã:Ca. Exchanges of geminated vs prenasalized consonants also occur in Semitic: manṣaru = maṣṣaru "guard, keeper", but there it does not lead to nasalized vowels.