Coinbase had said they would add BCH to their exchange before the end of the year months ago. And people had noted BCH appearing in their API a day or two ago, and then the trading pages were up for an hour or two before the official announcement, with some very large orders being placed in advance of trading going live, with the price already set at $1000 dollars higher than any other exchange was offering.
Add to this the fact that anyone who held BTC on Coinbase in August receiving BCH, plenty of people still holding BCH from the last time the price spiked, and the exceptionally deep pockets of certain people/groups who are supporting BCH.
You didn't need to be a Coinbase employee to know this was coming, and that it would, at least short term, have a significant effect on the price. Crypto traders live on big news and swings like this, and it was inevitable that it would be a big event.