You seem to assume that a) the coinmarketcap is a measure of the actual market value, and b) that therefore the coinmarketcap can not perform sudden leaps.
Consider this reductio ad absurdum: what if I create another fork of Bitcoin (e.g. Murchcoin), and sell one Murchcoin for 10,000 USD on some exchange (probably to myself, because nobody else would be interested). Did I suddenly create billions of market value? And yet, the coinmarketcap would show that Murchcoin has thrice the current "market value" than Bitcoin.
We therefore observe the following:
- Coinmarketcap is easy to manipulate. If you go through the top 50, you'll find multiple coins and tokens that resemble my Murchcoin example from above.
- Coinmarketcap is at best an imprecise indicator for market value. For example, if someone tried to sell 100,000 BTC in one day, surely they'd move the price, if not crash the market.
- A coin and a deriving forkcoin do not necessarily have to have the same value in sum before and after the fork. E.g. the emergence of BCH might have created value by reducing uncertainty: It gave two incompatible visions of what Bitcoin should be a way to coexist. It made other fork projects (Classic, BU) less likely to happen independently by gathering many of their supporters.
We also observed that there was a lot of trading of BCH in the weeks after the fork. However, this resulted mostly in more demand for Bitcoin and drove the price up. This confused me at first, but I surmise that some capital that had been waiting on the sidelines for the scaling debate to subside took the fork as a signal to move in. Hereby, I assume that next to the fiat that moved either into Bitcoin or BCH, many of those that sold BCH took the revenue into Bitcoin.
In conclusion: there was already a community following and demand for BCH, which is why it established a relatively stable price. More money entered cryptocurrencies at that time. And coinmarketcap alone is an imprecise metric, it should at least be combined with other metrics such as e.g. trade volume.