なぜなら(naze nara) appears at the beginning of a sentence just to signal that a reason is given in the following sentence. It is always paired with some form of "because" at the end of the sentence such as からだ(kara da), から(kara), ので(node) etc., and なぜなら(naze nara) itself is not obligatory. Related to なぜかというと(nazeka to iu to) and somewhat less formal.