Celtics made a big mistake with latest trade

What am I missing? Obviously, we are not privy to everything that goes on inside an NBA organization, and I wouldn't expect to know the inner workings of the Boston Celtics' front office, but what were they thinking?

Even if you don't always agree with a transaction, in most situations, you can generally understand the why. However, with the Celtics trading Jaylen Brown to the Philadelphia 76ers for Paul George, the why escapes me.