an informal phrase meaning 'the reason is that...' or 'because...', used to introduce an explanation.
なんでかというと、明日急に用事ができて旅行をキャンセルしたんだ。
The reason is that something urgent came up tomorrow and I had to cancel the trip.
彼が来なかったんだ。なんでかというと、電車が遅れていたかららしい。
He didn't come. The reason seems to be that the train was delayed.
説明を省いた。なんでかというと、時間がなかったからだ。
I omitted the explanation. The reason is that I didn't have time.