I get the feeling Alexis felt it was time to move on and this was the perfect opportunity. I know others will disagree with my sentiment but having been apart and then later an observer of the Reddit community I can see why he would want to move on and do so as gracefully as possible.
I have kind of a weird opinion of this. I actually think Michael Seibel is too quaified for that position. What we want in technology is more diverse voices in leadership positions, not more of the same voice in multiple places. Obviously he's a great voice for Reddit to have on its board, and more diverse voices at reddit is good, but I do worry that we're still fulfilling this idea that the standard we're holding non-(straight-white-male) people to is actually higher because we're scared of being accused of hiring someone as a diversity hire.