Presumably a combination of reanalysing "alumn-us", "alumn-i", etc. as "alum-nus", "alum-ni", etc. and a phonological preference/constraint against word-final /

Presumably a combination of reanalysing "alumn-us", "alumn-i", etc. as "alum-nus", "alum-ni", etc. and a phonological preference/constraint against word-final /mnz/. And against /mn/ too: even English column is /ˈkɒləm/ and not /ˈkɒləmn/.