The leak was plugged, his name was Seth Rich.
That’s what they are covering up.
Assuming that is true, why? The DNC emails were at worst embarrassing. They showed the DNC was partisan, but anyone with 2 brain cells could tell the DNC was rigged. Bernie never stood a chance and it had nothing to do with collusion and everything to do with the way the DNC counts delegates. The DNC leak did not derail her campaign. I don’t think that the DNC leak even made the list in her book of reasons for losing.
If they were willing to go to such lengths to cover up a small embarrassment, I shudder to think what they have done over much more important topics.