The part in Mueller’s report about Russia hacking the DNC makes the criminals think they got away with Seth Rich, but there was a previous hacking, supposedly by Guccifer, that gave SR the idea of downloading and getting the email files to Wikileaks, so the report is probably referring to that. Also, Podesta’s hacking with a usual phishing email may have come from Russia, but not the main DNC leaks that were proven to have been downloaded onsite.

Because they think their crimes are still being covered up, and because they trusted false evidence that Trump’s children were being indicted, they want to look for more dirt. And dirt they will find, and much worse.