Why is デレる ichidan? (Etymology of デレる and デレデレ)

Question

I thought modern slangy verbs coined from onomatopoeia and loanwords were all godan verbs (for example, ググる, コピる, ピヨる, ボコる). This has been asked several times:

• What are the principles behind turning foreign language words into verbs?(e.g. ググる and サボる)
• Characteristics of 'loan word root + る' verbs
• Can you form verbs from 擬態語 or 擬音語 by adding -る?

However, I recently noticed デレる might be an exception. In the general perspective, as this article suggests, デレる is a recent slang term coined from ツンデレ/デレデレ/デレ in the last few decades. However, it conjugates as an ichidan verb (デレない, デレました, デレた, デレろ rather than デレらない, デレりました, デレった, デレれ)!

Why is this? So far I have two hypotheses in mind...

1. Is this because でれる is actually older than デレデレ, like 照れる is older than テレテレ?
   I found there was an example of でれる even in the meiji era. However, according to 青空文庫全文検索, デレデレ was a common word 100 years ago, and でれる was virtually nonexistent in those days. Still, I have no means to check how these words were used pre-Meiji. (Is there a public corpus for classical Japanese?)
2. Is this because the stem of デレる ends with レ?
   Most (all?) verbs that end with れる are ichidan verbs (e.g., 入れる, 荒れる, 枯れる, 割れる), and デレれ does sound somehow weird to me. So it's not surprising if there is a rule that verbs ending with れる must be ichidan even if they are from loanwords/onomatopoeia. But I cannot think of any similar example (or counterexample) of this.

As an amateur language fan, I could affirm or negate neither of these two hypotheses. I believe I am not the first person who noticed this, so there may be a research about this. Any suggestion will be appreciated.

Answer 1

My current understanding is as follows (thank you Yosh and broccoli forest for the insightful comments).

デレる is an ichidan verb probably because it's actually an old verb coined in (or before) the 19th century, when the verb-coining rule was different from that of today. でれる did appear in several works in the 19th century. Although でれる was rare according to 青空文庫, it was common enough to be listed at least in three dictionaries. Several slangy verbs coined in this period were ichidan, and ぐれる (ichidan, "to turn delinquent", from ぐりはま) is a good example of this.

Hypothesis 1 is probably not correct because all the dictionary entries I checked so far explain でれる based on the meaning of でれでれ, which implies でれる derived from でれでれ, not the other way around.

Hypothesis 2 is not correct, either, because there are several more recent godan verbs whose stems end with れ:

• ナレる: godan, "to narrate", from ナレーション (narration)
• ロレる: godan, "(for a pronunciation) to be unclear; to slur", from 呂律