The most likely explanation with McClinton is that he signed last week with a long shot of making the team. The addition of Bogans turned this long shot into no shot at all.
Having no shot of making the team, McClinton asked to be released or Spurs offered him to be released to try his chance elsewhere (other NBA training camp or in Europe).
McClinton twitter "big decisions to make right now,,, reall big decisions,,,,," makes me think that is was the second option and Spurs have been honest and nice with him.