If this sentence was instead「なんの理由あるか分からん」I would understand, but I don’t get why あって is used here instead.
I’m by no means an expert, but some times characters use って as a speaking style. I know in Doctor Stone, Senku uses it a lot. So it might just be something like that.
It could just be a quirk, Araki is known for playing with the Japanese language a lot. You would probably have a better chance at seeing if Jonathan’s dad has spoken like that before, but either way, I think it’s just emphasis on the connection between how much he does not understand the reason why Jonathan would be doing what he’s doing. It reminds me of the だって or って type of speak that would replace the は in a casual sentence, like, “as for what reason you have, I have no idea but…” (but is added since the original text has a が）
I do not personally believe it changes the translation in a drastic way, and it would still be the same overall meaning if it had used あるか instead of あってか. Of course I could be wrong, but when it comes to Araki Japanese, he’s famous for his voice in his Japanese works for being a little off the regular path so
Your question gave me a kind of existential crisis trying to figure out what the purpose of ～て＋助詞 is, so my answer is going to be kind of disorganized due to it being in the order of research I did (I searched Google for grammar help in both English and Japanese). Here’s what I found out and keep in mind I’m not 100% sure in my conclusion though it seems logical:
あって＋か is a grammar pattern used in plain/casual speech (the sentence will end in a plain verb).
It can be used in sentences like:
そんなわけあってか → Meaning = そんなわけ（が）あってか（どうか分からないが） → Translation = “Probably because of that”
そのせいもあってか → Translation = “Probably/maybe because of that”
I remembered that there are other times where ～て goes in front of a particle (ex: ～ても / even if ~、～ては / when ~) so I thought researching about the purpose of the ～て form might help.
～て connects two simple sentences into one sentence. The meaning isn’t as blatant as other grammar patterns (ex: ～ので or ～のに obviously give a reason) so you have to figure out why the two basic sentences are being connected.
It has 11 functions based on this website (「て形」の１１の機能 ＋ 「ている」の機能 | 日本語教師のN1et):
- Actions conducted in succession (ex: 私は仕事を辞めて、大学に入りました / I quit my job and went to college).
- In parallel (ex: 安くておいしい / cheap and tasty | 大きくて、重い big and heavy).
- Comparison of plural matters (ex: 兄は買い物に行って、弟は掃除をしました / Big brother went shopping and little brother cleaned).
- Incidental situation (ex: 朝ごはんを食べないで学校へ来た / Came to school without eating breakfast)
- Means and method (ex: 大根を買って、おでんを作ります / Bought daikon and will make oden → Meaning = 大根を使っておでんを作る / Will make oden using daikon).
- Showing cause/reason (ex: 手紙をもらって、嬉しかったです / I got a letter and was happy).
- Telling the starting point (time or location) (ex: 学校を卒業して、２０年経ちました / Graduated school and 20 years went by → Meaning = It’s been 20 years since I graduated school).
- Contrastive conjunction (the relationship between the two basic sentences isn’t logical) (ex: 彼は知っていて教えてくれない / He knows and (but) he won’t tell → Alternative way to say = 知っているのに / Even though he knows・知っていながら～ / Although he knows).
- Expressing concessions and limitations (ex: 安くて１万円くらいだろう / It’s cheap and probably around 10,000 yen → Meaning = The cheapest would probably be about 10,000 yen).
- Looks like something/Gives the impression (Mainly using ～て見える) (ex: あの人は老けて見えます / That person looks old)
- Have a purpose and carry out consequential logic (ex: 彼はアメリカを目指して太平洋を渡った / He’s headed for America and crossed the Pacific Ocean | 私は彼女をかばって銃弾を受けた / I protected my girlfriend and took a bullet (was shot)).
So I guess with all that said,
なんの理由あってかわからんが = 何の理由（が）あってか分からないが…
I think the lower examples on this list seem to suit 「なんの理由あるか分からんが」 because it’s contrastive (there is a reason, though he does not know what it is). And signaling that contrast is what ～て is doing.
Edit: I forgot to say that I did find other examples of あって＋か (it seems to be a grammar pattern used in plain/casual speech, the sentence will end in a plain verb):
そんなわけあってか → そんなわけ（が）あってか（どうか分からないが） “Probably because of that”
そのせいもあってか “Probably/maybe because of that”