My understanding is that Garland's team (Klutch, FWIW) wanted Garland out of Cleveland if Mitchell re-signed. Klutch's view is that Mitchell's presence will devalue Garland by taking away touches and playmaking. Which is a sad way to view things, but as everyone says, "this is a business". I initially got the sense that there might be some personality clashes, but I think it's not really that. Really just Garland would rather run a team than be a perpetual second fiddle to Mitchell.
The crazy thing, I am reasonably sure that Mitchell finishes his 3 year extension somewhere other than Cleveland. I agree with the other posters who said that Mitchell took the money offered to him because he could always agitate for a trade later. I will not be surprised at all in a year when Mitchell is shipped to the Heat for some combo of Herro/Robinson/Rozier, maybe some filler and a few picks. If I had any sympathy for the Cavs, it would suck that they could lose out on both guys in short order.