I think I solved this. I believe the problem was EMI (electromagnetic interference).
After putting ferrite beads on all the signal and power cables and rerouting the cables, I’ve now been running 4 printers for 3 days without a halt.
Here’s the video I used as a reference for understanding and protecting against EMI in the context of 3D printing.
This was a hard problem to troubleshoot, due to its seemingly random, intermittent occurrence. I hope this info can expedite a fix for anyone else experiencing the same, when running Klipper via USB cable.