You're definitely right that BTC has maintained a high correlation with other cryptocurrencies. Historically, this has been because, if you wanted to buy an altcoin, you had to first buy Bitcoin and then run it through Shapeshift etc. What's unique here is that BCH has specifically been rising while other alts, even Ethereum, have stayed comparatively flat. This implies that the rise in BCH over the past few days isn't due to a general rise in all altcoins but is instead due to traders specifically moving funds into BCH in relative isolation.