Sounds like the agent wasn't part of it, it was all DJ and Blake.
Don't blame the Clips, FYI. I just blame DJ.
I put the bulk of the blame on DJ. Not because he changed his mind, that's his right. Looking at it from an employee/employer perspective, it's not like he's the only person in history to want to change companies and then decided at the last minute to stay put (he's not even the first NBA player to do it). However, when he decided he was staying in L.A., he should have immediately called Cuban (or at least answered the fucking phone) and told him "I'm sorry, I've changed my mind.
I also put part of the blame on the Clippers. Not for re-signing him. If he wanted to come back and they still wanted him, they should absolutely re-sign him. They'd be stupid not to. However, as a professional organization, they should have either left for a couple of hours and allowed Cuban the meeting (which apparently had been agreed to), insisted that he call Cuban and tell him or, at the very least, Ballmer should have called Cuban and explained that DJ has had a change of heart, but doesn't want to talk, so he's calling on his behalf as a courtesy.