Except a few days ago he had no thoughts of resigning which signals a possible forced resignation
Or
A sudden catalyst
1. The CNN story says these indictments were handed down from a D.C. grand jury.
2. The Flynn investigation was apparently being done through a grand jury in Virginia, since that Federal district has jurisdiction over CIA and national intelligence cases.
3. Boente was working as the U.S. Attorney for the Virginia district, and was assisting Mueller in that aspect of the investigation.
Putting two and two together here, I'm speculating that the CNN story and the Boente resignation tells us two things: (A) the indictments (if there are any) don't involve Flynn; and (B) Boente's resignation (and the lack of any urgency in his departure) may indicate that his work on this case is done, and the Virginia grand jury never handed down any indictments at all.