It may seem initially slightly surprising, but there are necessary truths that are explained by contingent ones. For instance, it is a necessary truth that Obama is president or 2+2=4. And this necessary truth can be explained by the fact that the majority of the electoral college voted for Obama, or, perhaps even better, by facts about the way the Democrats and Republicans campaigned. Another necessary truth that can be explained in the same way is that it is or is not the case that Obama is president.
That the necessary can sometimes be explained with the contingent is, I think, a rather more trivial claim than that the contingent can be explained with the necessary.