I'm not too worried about the witnesses powering down; it seems to be a perpetual cycle, I expect primarily to pay for the expense of running a witness (servers, bandwidth, storage, etc.) — when the price of Steem is higher, they power down less, when it's low, they power down more.

Another possible explanation for the increased volume on Bittrex could be that Huobi and Binance both have their Steem withdrawal gateways suspended due to the fact that Justin got the Steem they have in client trust accounts locked up for 13 weeks... and people are simply using a different platform to trade.