Apparently, it's because Jerome Corsi is connected to Roger Stone. Stone is an associate of Trump, apparently, and works with the National Enquirer. I assume this is Mueller following up on Cohen's guilty plea to "campaign finance" charges that involved shell corporations and payoffs to bimbo eruptions.
I use the term advisedly. Clinton was credibly alleged to have groped and even raped people, although his most famous affair with Lewinsky was consensual. Trump has been accused of having consensual sex and paying the women to keep quiet about it. When they don't, that's a bimbo eruption. Sorry, Stormy.
In any event, the odds that either Jerome Corsi or Roger Stone have anything to do with Russian collusion to influence the 2016 election -- which is Mueller's only area of authority -- seems very low. Mueller is likely looking for more "campaign finance" crimes he can use to coerce guilty pleas that purport to implicate Trump. And he wants those because he desperately needs leverage.