Okay, so as we all know, Leah is the first ever female werewolf...shape-shifter wolf or what ever. Before only men turned into wolves, then Leah appeared and no one knew why.
But after the guide came out which had some family trees in it, it turns out Leah and Seth get their wolf gene from both their parents. Their dad has the Atera gene and their mom had the Black and Uley gene.
So maybe since Leah and Seth have the gene from all three families, this is what caused her to phase because the wolf blood is strongest in her and Seth.