Couldn't say the cause but apparently, the chart was triggered after the release of 0.16.0 and not Coinbase segwit support (which happened a couple of days before).
Maybe more smaller services had it implemented besides Coinbase. But I do know Bitfinex is also enjoying Segwit right now from their this tweet. Hehehe.
https://twitter.com/bitfinex/status/969255478550319105
We are working hard to reduce this number further in the upcoming weeks and we are excited by the improvements made possible by SegWit.
Segwit transactions have also increased to 30%!
Finally. Bitcoin is moving forward and will prove that there is no need for Bitcoin Cash.