五段動詞特異的なイ音便について


Are there any explanations for the 五段動詞-specific イ音便 of 連用形+た/て that aren't
based/dependent on the 子音語幹/母音語幹 explanation for the differences in 五段動詞/上一段動詞
conjugations?

For example, take 開く and 飽きる, setting aside the fact that they have completely
different meanings and focusing only on their pronunciations. In 口語, the 過去形
of あく is あいた, due to the イ音便 of "あき+た". However, for the 上一段動詞 "あきる", this イ音便
doesn't occur, and so the past tense is "あきた".

I've heard explanations regarding 子音語幹 and 母音語幹. However, they don't seem able
to explain why the イ音便, which occurs based on how difficult a word is to
pronounce, would not occur indescriminately. Is anyone aware of any other
explanations?

In other words, if the past tense of 開くwas reanalyzed as 開いた, why was the past
tense of 飽きる never reanalyzed as 飽いた？




I'm very curious how you came across the idea that "イ音便 occurs based on how
difficult a word is to pronounce".

Regardless, as far as I know イ音便 occurs solely with 五段動詞. As others have
mentioned, "飽いた" is conjugated from 飽く（あく）, which is the regional/somewhat old
variant of 飽きる（あきる）. The fact that 飽きる, the 一段 version, evolved into
mainstream use is simply a matter of history.

My guess is that before the popularization of kanji, the pronunciation of the
五段 verb 飽く overlapped with other words that had different meanings (空く: to
become empty/vacant, 開く: to open/to create a gap, 明く: to come to an end/come
to light) which caused confusion in speech, leading to 飽きる becoming preferred.

But then, you also have words like あげる which, against all odds, has
accumulated nearly 30 different meanings. The three kanji used to
differentiate between those meanings is hardly adequate.

Again, a matter of history.

[![上げる・挙げる・揚げる](https://i.stack.imgur.com/sfzMb.png)](https://i.stack.imgur.com/sfzMb.png)

