I had this problem today:
I was under the sun like for 5 minutes, 14 degrees here, the phone got a little warm but nothing compare with while charging or heavy gaming and then it restarted and I got a excessive temperature on the USB port warning.
I had nothing connected to the usb, not even the cooler.
I was only using a music app at the moment.
I left the phone to cooldown inside my house like 5 minutes, I restarted it and no issue anymore.
Anyone experienced this?