Why is my circuit in parallel with a voltage regulator overheating?
It happened when you plugged in the USB? That seems to suggest it created a short to me. You can probably test that with everything powered down. Plug everything together without the DC input and check with your multi-meter (although, you probably need to test with a board that isn't burnt out).

In that case you need to swap out that buck converter board for an isolated one. Really you should be isolating the Pi from the other board any way. Having them on a common circuit and then making another connection between them (USB) strikes me as pretty risky, especially without having any idea how the printer circuit is designed.