One thing would be to get more BTC with your BCH, another would be to sell part of the BCH into dollars, and another (that you are assuming) is that they are selling more than the BCH dollar amount, which is a bit larger of an assumption....
Does not have to be more. If BCH is being pumped with BTC but the pumpers are using older saved BTC and the sellers of BCH are converting to fiat it could put substantial downward pressure on the Bitcoin price. This is what I think is actually happening but that is just speculation.
Having your funds in long term cold storage can make for a fairly powerful psychological barrier to selling. As holders breach cold storage on a large scale to cash out BCH it will probably will push some people to sell who would not otherwise. How much and how relevant this will be is unknown.
Honestly, given these negative pressures I believe the simple fact that the price has not totally crashed is a very bullish indicator over the medium term. I put my money where my mouth is and I am now 1% fiat 99% BTC and 0% BCH so perhaps that makes me biased
