Here is my theory - posted here so people can pull it apart and make me smarter by pointing out my errors !
Like it or not, Coinbase / Gdax is a primary fiat gateway. Fiat flows into crypto through Gdax and also flows out through Gdax. During the rush to $19k, Gdax was often $500 ahead of Bitfinex and Bitstamp who had to be dragged kicking and screaming through arbitrage upwards.
Now we see a $500 gap in the other direction. Bitfinex is trying to soar but Gdax is dragging it down through arbitrage. The theory is that there is a net outflow of funds from crypto.
The test for my theory should be that the order books on Bitfinex should be shrinking - I have not tested this yet.