USD was the largest or second largest market in bitcoin and cryptocurrency in 2017. There is about $25 billion (estimated) to be owed on capital gains from 2017 alone. While that doesn't explain all the selling, it likely has been a contributing factor. Either way, we will know for sure in about a week. Tax day is right around the corner.
Regarding the futures markets, there is a bit of coincidence that since they have launched, prices of bitcoin have gone pretty much straight down. While there hasn't been outright visible manipulation on those markets, they could provide the avenue for those to place bets on moves to the downside, while manipulating them in the spot markets. Just some thoughts...