I'm pretty sure it was Gray that made the move. CBS was making a lot of money in fees from WANF to be an affiliate, and didn't have to do much work to get it. Aside from a dream of setting up a streaming OTT service in the largest market in the southeast, CBS probably had little motivation to take the affiliation from WANF and turn WUPA into an O&O. Especially at a time when their parent company is planning a merger, and their news department is in absolute chaos. Not to mention having to build a local news department from nothing in two months in a very competitive market. I don't think CBS wanted to make this move at this time.
Meanwhile, Gray, which has recently been building up a competitive news operation at WANF, has been paying millions of dollars for a CBS affiliation that apparently hasn't been paying off to their expectations. The network news department is a mess, and they no longer have rights to SEC sports. Without CBS, they save on paying for the network, and they can go after more lucrative local sports rights, which may become more abundant in the Atlanta market. They also can expand local programming, including news, where they collect 100% of the advertising money. They have some flexibility here.
The new CBS Atlanta operation is going to be a huge money pit for a long time, at the worst time possible. Sure, they can put on a happy face and spin it, especially when it comes to the new streaming news channel and a deeper presence in the South. But I highly doubt this was their intent.
I've been reading lots of ridiculous speculation and conspiracy theories on other boards about what this means for Seattle and Tampa Bay, where Paramount/CBS owns independent stations, but affiliate the network with other broadcasters. I really can't see Paramount/CBS wanting to take affiliations away from Tegna or Cox to create their own startup O&O operations on much weaker stations, when they could just merely sit back and collect the reverse comp money from the current, well-established affiliates.
If Gray thinks they can make a go of a news-intensive independent station in Atlanta (after the fashion of WJXT Jacksonville), more power to them, after all, it is the nation's seventh-largest TV market. From all I read, I just assumed that CBS dropped the bomb on them. And it's entirely possible that some of WANF's news talent may migrate over to WUPA, viewers like familiar faces doing their news, and a de novo news operation needs that. It's just awfully intrepid of CBS to think that they can create a viable news operation from scratch in a market where you have three legacy stations, and a newly-independent station that already has news and isn't going to back down from it. Gray also has formidable news resources, so if anyone can pull it off, they can.