One very interesting point from the post race pen.
Toro Rosso had told Hartley to give Gasly the position at the hairpin.
Hartley proffered the information first. Gasly confirmed it.
When he saw Hartley lift and coast he assumed he was acting on order to switch.
The rest we know.

https://twitter.com/wbuxtonofficial/status/985432943811289089