What USB Accessories?
I haven't seen the popup again but I wouldn't know what it referred to. No new accessories are connected.
iMac (2017 – 2020)
Question marked as
It means that a USB device or combination of USB devices has caused the USB port(s) to overload and shut off.
If no new devices have been added, then it could also mean that you have a failing USB device or cable.
If it only happened once, then it could have been a fluke and there is nothing to worry about.
If it happens alot, then you will need to do some testing to figure out what device or combination of devices is causing the problem.