Final feedback and solution
The issue happened again, and at the same time I was seated at my desk:
Setup:
Macbook Pro /w Kensington Desktop Hub to provide DisplayPort and Ethernet
HP Desktop PC with Windows 7
Polycom Desktop Phone
HP LaserJet Pro
Wifi Access point
All the above are on the same network switch
Event
I disconnected the Kensington device from my Mac (USB-C)
I noticed something was wrong when my phone reported no network (It makes a chime when this happens)
My Windows 7 box was doing a download and then said the internet was disconnected with high CPU usage.... hmmm....
My phone complained about no internet (Whatsapp went offline). Had to use the regular mobile LTE network.
Root Cause
When the Kensington unit is disconnected, it goes into a state where it jams the upstream network switch, it appears to flood the LAN with rubbish packets... the switch falls over or locks up. Also other devices see (and receive) these packets.
Left to happen long enough, the OrangePi will crash as I have described. My colleagues have been able to reproduce this, and we've decided to throw the Kensington unit out.
It is not a bug! The desktop phone also eventually crashes, and it takes out my Ubuntu 18.04 LTS file server eventually. The HP printer locks up completely and I have to power cycle it at the mains socket.
Sonikku's post in System crash with network manager being the cause was marked as the answer