The reason for his disquiet is, presumably, that this scandal has the potential to derail Harper's momentum towards attaining a conservative majority in the near future. Such is the influence of Mulroney in the Conservative party that any action against him will cause distraction and dissension in the party.
It could go further. Some prominent party members, such as Peter MacKay, are so closely entwined with Mulroney and with supporting him in this scandal that they could go down when all the facts come out. Schreiber hinted at coverup when he said that he had sent a letter to Harper months ago that outlined what is known to date. The coverup could go far, far deeper.
In fact, Schreiber, who is a smart man with a smarter legal team, seems to be initiating some sort of shakedown. He has been telling reporters that he has more tales to tell and more evidence to reveal. My guess is that Harper now knows that his government is implicated.
Perhaps Schreiber's goal is to delay his extradition to Germany by making himself indispensable to an inquiry in Canada. Perhaps he is hoping to blackmail the government into something more. What we know about him is that he has a long history of extremely dubious dealing; he gave Mulroney a significant amount of money but feels that Mulroney reneged on his end of the deal; and, facing the rest of his life in jail, he has very little to lose.