DC DC Converter Failure Modes
MOSFETs tend to fail closed. A well designed DC-DC converter might use it's low side FET to cause a short to ground if the high side FET starts to fail. If so it would be depending on a properly sized fuse to interrupt the current in this sort of event.

If the driver doesn't detect the failure, you might just see the converter get less efficient and run hotter causing further damage to the system.

Capacitors like the output capacitor can also fail and so an older converter is likely to have less effective smoothing on its output.

All that said, 500mV ripple shouldn't kill a USB device like that. Perhaps the excursion that did, was more severe than the ones you observed (possibly because the electronics had gotten hot or were operating at an increased load.)

Check the lifetime on your capacitors and see how it compares to the operating life. Schedule their replacement if you have a system that wont tolerate their failure.