After watching the broadcast they did this morning to talk about it, I can probably clear up for some people who weren't following closely.
Basically, there were creative differences building for a long time over what the direction of the group would be, with Colin pitching hard-hitting political projects (both relating to games and not) to contrast the sillier, lighter tone the other three were going for. Colin had recently launched a successful talk and interview show, Colin was Right, which was gaining traction. Meanwhile, he started to appear on other shows of note, including the Rubin Report and just this morning, Glenn Beck.
The differences came to a catalyst last week when Colin tweeted a joke at women's expense on International Woman's Day, which sparked a controversy, but perhaps not quite so severe as the controversy over Greg Miller's official KF response to the joke, which many saw as the other three throwing him under the bus.
Rather than fester a growing divide in the audience, it seems Colin decided to resign so he could pursue his own projects and speak his mind without worrying about how it would affect the livelihood of his friends (and they emphasize that nothing on their end proceeded with bad blood or anything). It's unclear what he is doing next.
tl;dr: Colin resigned in good faith over creative differences and to say what he wants without consequence to other people he cares about. The controversial tweet was not the reason, but it is probably why this is happening now and not later.
