Subject pretty much explains the problem. One can just get power from a USB interface, but if I plug our nRF52840 into such an interface, it does not appear to boot into its application. I see the main LED blinking rapidly. When I subsequently plug that board into a USB interface on a computer, it starts the application.
These nRF52840 boards should work and boot into their application regardless of whether the USB interface connects to a computer. The application may be a mobile app, and if so, it should not have to be connected to a PC. Instead, it should work off a battery pack with a USB port.