Looks like that article was written on the 23rd of December.

That's the same time period around which Coinbase abruptly released Bcash. That time period was a barrage of negative kerfuffles for them.

Historically Coinbase/GDAX has enjoyed "inflated" BTC/USD prices because of their trustworthiness. Contrast with Bitfinex which does not serve US customers, has been hacked in the past, and is (last I heard) in debt from said hack. Or contrast with any of the other exchanges which are either questionable or located outside the U.S. Gemini is the only other state-side trustworthy exchange besides them.

(I say inflated, but it's probably more appropriate to say that the price is a discount on all other exchanges; a discount which accounts for the counterparty risk and difficulty moving USD in/out of them.)

The late December fiascos that Coinbase suffered has likely shaken that sense of trust, at least in the short-term, resulting in both reduced trading volume and reduced prices relative to other exchanges.

So the question is, which theory is more likely? That a single trading bot has complete control over Bitcoin's price and its discovery has resulted in the dismantlement of the program? Or that traders are reacting to the news of "insider trading" by Coinbase employee's? Or maybe it's as simple as being the end of December. The winter holidays slow down markets.