It seems that yesterday at around 01:00 UTC, someone with a huge amount of cash on GDAX started a buying bot, which bought BTC in small increments every second or so at market price. Judging by the GDAX volume in comparison to normal volume and an estimated average price, I'd say this person bought about 50-60k BTC for around 800-900 million dollars. Shortly after this buying activity started, the Bitcoin network mempool got clogged by transactions - and it doesn't seem like spam, but more like legitimate transactions with fees attached that would normally go through in an acceptable timeframe. I'm pretty sure this is people realizing that GDAX is the place to sell right now, thus they're trying to move BTC from other exchanges and from cold storage onto that exchange. However, most of them likely didn't get there in time, but the buying bot apparently had no upper price limit and just continued buying its way through the order books, until finally arriving at 19,6k$, when someone apparently hit CTRL-C.
During that time, the price on GDAX and all other exchanges decoupled, as arbers probably gave up on arbing (guess they ran out of coins or cash respectively and weren't able to interchange the funds between the exchanges, just like all the sellers). Even the Koreans, which usually are paying a large premium with regard to Western exchanges, almost lost all of their premium over GDAX for a few minutes.
I have only one idea who could have caused this: a single person or entity trying to load up on huge amounts of coins, planning to use them with corresponding orders on the upcoming futures markets. They probably got surprised with the CBOE announcement of starting futures even a week earlier than CME and thus had to speed up the buying plans a little, regardless of the premium they'd have to pay this way. The tricky question that remains however is: are they going to use these coins as a hedge, or are they planning on dumping them to do the inverse of what they just did (artifically lowering the market price, so their shorts on the futures markets earn them way more than they lose on the BTC exchanges)?
And of course: is this over now? GDAX went down and up again, and the mysterious and frequent buys at ridiculously high prices have returned as well, although the blockchain had practically no time to re-stock GDAX with coins. This person definitely doesn't care at all about the price and just wants to load up on any coins it can get really, really quick.