where it is coming is in your question only. The Bitcoin Cash was the first rival in the to grab most of the bitcoin's market cap in very short period of time and that why we saw major drawback of bitcoin going down. Now when the community has reverted the investment, the things has gone upside down and the major (currently) market cap is coming from the BCH sell only.
Also, in other ordinary day obviously the market cap is being produced by our real hard cash which most of the investors are trying to double or triple here in the crypto chain. That is from where the actual money flows into bitcoin and why wouldn't it be flowing like that after seeing such great giant leaps here.
To be honest ETH was the only coin that truly got once close enough to BTC to challenge dominance, for BCH everyone knew that it was a pump and it was doomed to fail sooner or later.The difference between those two coins and the way they grabbed a piece of the marketcap is enormous, at least ETH is still stable after a surge BCH is dropping like a stone and ETH has some purpose, BCH is just....another altcoin.
As for the money that flow into BTC I wouldn't trust the marketcap index.
During the fall last week the marketcap went down more than the actual trade volume which is pure nonsense.