Timing is everything. Wait for an enormous disaster to sneak through something you KNOW you shouldn't do. Who is gonna notice it?

Trumps ego (maybe) refuses to be upstaged by a serious hurricane...      (just speculation)

For people of sound mind/s: (Trump excluded)  The term "Friday news dump" can apply to the world of politics, business, and even sports. The news in a "Friday news dump" is usually seen to be negative towards the entity that is releasing it.

For instance, a ruling political party may release a few pieces of negative news on Friday evening. The reason for this? Many people don't pay attention to the news on Friday evening and Saturday morning, so a "Friday news dump" is a good opportunity to slip negative news out to the public without too many people noticing it. 

All administrations have used the Friday evening news drop often on holiday weekends. Just part of the game they all play.

Perhaps Trump wants everyone, especially Mueller, to know that he has the power to pardon federal crimes.