Most USB ports have a chip between it and the host controller for electrostatic discharge (ESD) protection. The nicer ones can also sense shorts and overcurrent on the lines and signal the processor that something is up.

Example: https://www.maximintegrated.com/en/products/power/protection...

Maxim integrated really has a chip for everything

And availability for nobody, even before the pandemic. I always cringe when someone specs Maxim. It's impossible to get eval kits.

>[...] but I guess it senses a short across some of the pins inside?

Most lithium ion chargers have (or should have) some kind of short circuit protection. It would make sense that the internal charging unit could make this information available to the operating system.

My wife was at a meeting, and a friend's kid spilled water all over the table they were sitting at (her phone was sitting on the table). Same thing - Pixel 4a indicated it wouldn't use the USB-C port until water or debris was removed. I used a Q-tip a few hours after the fact (she had cleaned it to the best of her ability), and that seemed to do the trick.