It happened with the hardfork yesterday, I don't think it was really announced anywhere though.
From what I gather, there was a lot of unintended side effects from the swap to 12 hours (when it happened uh, two weeks ago?). Nothing major, but generally it was perceived as a bad change.
The reason they went to 12 hours was to make the homepage feel less "stale". They decided that this wasn't the best fix for the stale problem, so they reverted it back.
Hope that helps :)
RE: Author and Curator Rewards Payout time has changed 作者和策展人奖励奖金时间已经改变