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？
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renyōkei
五段動詞特異的なイ音便について
Your question is understood in two part. One depends on the origin of the
word. Other depends on how people say it easily.  
Believe or not, Japanese dialects allows vary of 音便.

* * *

飽く probably came from old Japanese (OJ) words 呆きる(あきる) and 明く(あく) or they are
related. While people didn't familiar with Kanji's or writing, they remembered
words by phonic.

  * 飽く verb 1. satisfy 2. become bored/unwanted 
  * 呆る (OJ) verb 1. getting unclear what to do with it 2. being blanked.
  * 明く verb 1. to separate something with a big space or long time 2. to become unoccupied. 

They all mean 'no longer wanted' or 'had enough of that'. So that was alright
to be mixed together in the old days when people don't write the words and got
things done by oral.

* * *

Therefore, dialects are several versions and very much different in some place
especially with the cold climate.  
飽きた often is pronounced as あいた. I believe many of Kyoto and Kansai dialects
family (OJ users) prefer あいた, compare to 江戸弁 Edo-ben which has been spoken
around Tokyo prefered あきた, however none of them work in the Tohoku and Japan
seaside, it was going to be like French way, 'ai' makes 'e'. They often say
もう、えーた(もう飽きた)

No one knows what exactly was going on the word, I hope this will help you to
understand or bring you a bad guess to inspire the truth.
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2020-04-25T04:08:54.913
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answer
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)
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2020-09-27T14:07:53.117
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