We can’t know these things for sure. But it certainly seems like Mueller was sending a message and sending a shock to the Trump White House by releasing the Papadopoulos charges mid-morning. The Manafort/Gates charges are very serious as crimes and bring heavy punishments. They’re a big deal. But they do not connect directly to the campaign’s ties with Russia or even the campaign itself. The White House can accurately say that these are crimes that do not relate to them, as they have. (Set aside for the moment that they are serious crimes undertaken by the campaign chairman while the campaign was underway and that they provide Mueller with immense leverage to extract more information from Manafort.)
The Papadopoulos plea is quite different.
It shows a Trump foreign policy advisor in active communication with what appear to be Russian government officials or spies trying to get dirt on Hillary Clinton, arrange meetings with Russian government officials (even Vladimir Putin, rather ludicrously) and solicit Russian support. That an active foreign policy advisor was taking these actions while in active communication with the campaign about those actions is quite damning. An unnamed campaign official sent back word that a meeting with Trump himself was not happening.
Papadopoulos was arrested in July and has apparently been cooperating since. I see no purely legal reason why the news of his arrest in July and guilty plea in early October had to be revealed today, other than keeping the news from Manafort. One other potential reason is that one of the ‘campaign officials’ referenced in the Papadopoulos plea appears to be Manafort. It sends two clear messages. First, we’re not at all done with collusion and we’re making progress. Second, we arrested Papadopoulos in July and he pled out in October and no one knew. So don’t think you have any idea what we have.
This may be projecting too much. But in revealing the Manafort news early, giving time for the White House to respond as you’d expect (nothing to do with us or Russia or the campaign) and then following up by revealing this Papadopoulos indictment certainly has the feel of sucker punching the White House.