It's been going on for a long time as a means to avoid violation of the Third Commandment.
That's ironic, since the Bible, itself, does not spell God as G-d. When one speaks the word, one cannot hyphenate it. Simply to discuss God and religious matters is not taking His name in vain. Otherwise God could never be mentioned in churches -- or at all, for that matter.
I think it's a deliberate affectation people use to try to convey the message that they are somehow more reverent than the next guy.