なぜかというと details
Examples
the reason is — announces the explanation for the statement just made
今日は早めに帰るね。どうしてかというと、明日の朝がすごく早いからなんだ。
kyou wa hayame ni kaeru ne. doushite ka to iu to, ashita no asa ga sugoku hayai kara na n da.
I'm heading home early today. The reason being, I have a really early start tomorrow. casual
この方法をおすすめします。なぜかというと、失敗しても元に戻せるからです。
kono houhou o osusume shimasu. naze ka to iu to, shippai shite mo moto ni modoseru kara desu.
I recommend this method. The reason is that you can undo it if it goes wrong. neutral
弊社では紙の資料を廃止いたしました。なぜかと言いますと、保管にかかる負担が年々大きくなっていたからです。
heisha de wa kami no shiryou o haishi itashimashita. naze ka to iimasu to, hokan ni kakaru futan ga nennen ookiku natte ita kara desu.
We have done away with paper documents. The reason is that the burden of storing them was growing year on year. formal
Formation
なぜかというと、plain clause + から（だ）attaches to plain clause
Notes
(なぜかというと、朝が早い) leaves the explanation hanging, since
the から is what marks the clause as the answer.
Related patterns
Similar
なぜなら simply announces that the reason follows and belongs to essays and reports; なぜかというと puts a question in the listener's mouth first and then answers it, which is why it fills lectures and spoken explanations and reads chatty in a paper.
から sits at the END of the reason it marks, so the listener meets the reason before knowing it is one; なぜかというと comes first and claims the floor for as long as the explanation needs, which is what makes it useful for a reason too long to hang on a single から.
んです offers the sentence itself as the explanation and needs the listener to have felt the question already; なぜかというと spells that question out loud, so it can open an explanation nobody asked for — a lecturer's move, not an answer to one.
なぜかというと announces that a reason is coming and is a spoken, teacherly move; sentence-initial というのは just resumes what was said and explains it, and is what written argument uses — both still close on 〜からだ.
Lookalikes
The same というと ending on two unrelated jobs — 〜というと／といえば picks up a topic just mentioned and starts talking about it (京都というと紅葉だ), while なぜかというと's というと closes a question the speaker has just voiced on the listener's behalf, so nothing in front of it is a topic being taken up.