I agree that one purpose is probably for payment of operations costs - I believe I remember them saying that they hired 2 or 3 people to curate "The Diary Game", and salaries and other costs need to be paid. I guess that another reason is probably to grab back some of the funds that were invested primarily to fight in the February/March "Witness Wars" and aren't needed any more.

It's a difficult position for Steemit. If they hold the funds, they're accused of being too centralized, but if they sell them they're accused of abandoning ship.

Still, in the short term, it seems that there will be substantial new selling pressure on the price of Steem. I'm not thrilled about that, even if the long term prospects are (hopefully) still good.