They previously decided against charging this as a standalone crime because they didn't think it would hold up without a stronger charge to carry it. In other words this is one of those charges that they tack on after they've already charged something that's worth their time and energy. It's obvious that Bragg reversed the decision to appease progressives who are more interested in charging Trump with anything they can than worrying about how strong the case is. They just want to see Trump get arrested. Any conviction or punishment would just be icing on the cake for them. Bragg is going to have a hell of a time making the legal and factual case that Trump was covering up another crime, and he has to do that to have any hope of not being barred by the statute of limitations.