Likely because most people are taking all money from altcoins and putting into BTC in anticipation of the upcoming hard fork.When forks occur, you get 1:1 of the new Bitcoin for the same amount you had in the old Bitcoin. And since the fork is essentially free money, the more BTC you own prior to the fork, the more free money (new BTC) you’d have.So if you had 3 BTC now, after the fork you’d have 3 BTC and 3 Bitcoin Gold. People are pumping their BTC now because of this.