According to the interbank broking platform EBS, USD/CHF fell from .8870 to .8700 at exactly 5pm NY time and then rebounded immediately to .8880.
I’m still trying to find out what caused this anomaly but it looks for the moment as if it was legitimate trading.
There are some specialist traders who specialise in this sort of thing, driving the market just after the NY close when it’s particularly illiquid but this looks like a vacuum was briefly hit. I’m sure we’ll find out more as the day progresses.