I wish I could fully grasp that - what am I missing? If the trace was a little longer it may not resonate?
It's so obvious, it could only be impossible to understand.
Presumably, you have an unterminated or mismatched stub somewhere.
The length of the stub is inversely proportional to the resonant frequency.
There
cannot be any longer transmission line segments that produce that ringing.
So, on the list of suspects, the long PCB traces
are right out.
IC interconnects (trace taper/neckdown/fanout, if applicable; pad and bondwire equivalent circuits) could plausibly be on the list, but hopefully are taken care of by the manufacturer (pin models and internal terminations designed to tune that out), and are still on the short side (< 2mm).
Also, the impedance of the suspected stub should be fairly high, since the amplitude of the ringing is much less than the total height of the transition. (Or fairly low, if it's a series stub, but that would be harder to explain.)
The common element in all of this: the probe itself. Which looks like it has dimensions, and RLC equivalent specs, plausibly similar to the observed effect.
You should drop the probe (not literally, that would be expensive~) and use a direct wired approach as your gold-standard reference. This will show if it is indeed the probe itself. Standard RF techniques apply: build a microstrip resistor divider/attenuator alongside the trace, to tap off some signal and route it to a coax cable. Use a wideband balun to resolve differential mode waves specifically (or matched lengths of cables and difference it at the scope -- precision matching probably not being required due to delay adjust in modern scopes).
Tim