What's causing my buck converter's switching node to oscillate only some of the time?
But I've never seen this behavior before, so I'd like to know if any other component failure could cause this.


All three scope shots depict your buck converter operating in discontinuous conduction mode (DCM) and pulse frequency modulation (PFM).

The sine wave you see mid-switch cycle is the near-zero current remaining in the inductor resonating with the parasitics on the sw-node (your scope probe being a large one). The converter opens the low-side NMOS near the zero-current crossing to prevent negative current conduction through the inductor.

Perhaps, you are seeing an abnormally light load on the 3V rail due to faulty down stream components, thus, causing the buck converter to behave differently on this board.

With good scope probing you may see your 3V rail rise sag a few 10's of mV, but it most likely isn't ringing at 1 MHz (that's an artifact of how you are probing your circuit).