Frankly, it's clear what happened.
Hayward chose Boston. Planned to tell Utah + Celtics himself, on his own terms. Some idiot in Hayward's camp leaked it. Others confirmed it because it was true. Now, Hayward's agent is doing damage control because it's a terrible look to have something like this come out on twitter without first informing teams.
That's the only way this all makes sense. Credible media guys have cited sources plural in Hayward's camp--there's zero incentive for guys in Hayward's camp to make this up.