As near as I can tell, there are three possibilities:
1. Brad Stevens made an idiotic trade. The problem with this idea is that, in my opinion. Stevens is one of the best basketball minds in the world, and has consistently made moves (whether or active moves, or deciding to stand pat) that positioned the Celtics to compete for championships.
For me, his track record earned a lot of trust from me, even in the face of this shocking trade.
2. Stevens genuinely believes that George is a player that can help us win a championship better than Brown. Whether that is because of Brown's attitude, or his +- stats, or George's team defense, or George's playmaking and shooting, or whatever.
In this possibility, Stevens essentially sees George as a forward version of Derrick White. A "really good basketball player" to use his terminology. He thinks George will contribute more to winning over this next short window (2 years until George's contract runs out), and then they will reset their window again with a different roster construction.
3. There was a team out there that Stevens has been negotiating with, but that team was meh on Brown, and they definitely did not want the third year of his contract, because they were building their team around other players.
In this possibility, Stevens is continuing to negotiate a trade package around George's shorter contract and draft compensation for the player(s) he really wants that, in his opinion, will help them win more than Brown would have.
Personally, I think that most likely team is the Pelicans, who have Poole's contract expiring next off-season, and then Zion's and Murray's the year after that. In this theory, they didn't want Brown's 65m on the books 3 years from now in his final year when they are trying to rebuild around Queen, Fears, and the guys they draft over the next few years (they have the Bucks 2027 first round pick). I also thinks its likely they trade Zion and, if possible, Murray for picks.