I’m going to venture to say it’s rooted in classical Japanese and how it was influenced Chinese. Most modern NA-adjectives are made from kanji compounds, which indicate they either come from Chinese or Kanbun (“Chinese” created by Japanese nobles and officials). Eventually writing and spoken Japanese merged, so the Chinese loan words would have found their way into the common vernacular like French found its way into premodern English.
In any case, modern NA-adjectives come from TARI and NARI adjectives (TARI and NARI are old copulas, like the modern DESU). I don’t have access to my notes, but there was a division of some kind between the kinds of adjectives.
Distinction aside, like modern Japanese, CJ is agglutinative - you stack part upon parts to a stem to inflect the word and alter/build upon the meaning. Your -KANA adjectives may have come from one of the conjugated forms.
