Those are the questions we'll never be able to answer for sure. All we can observe are correlations and cannot draw certain causations from it. Ecosystems are complex and its variables change constantly. The appearance of wolves was one notable change, but not the only one. Others have suggested drought as a possible factor for the change in elk population.

Which one of the explanations is the truth, I can't possibly tell. Both sound plausible to a layman, and neither can be proven definitively.