DC-DC Buck Converter Failure Mystery
One possibility is that your DC/DC converters may be overheating or that their lifetime is shortened by operating at a high temperature. I've seen capacitors rated for 3,000 hours at 80C so it's not crazy to think that this might be an expected failure mode for an input or output capacitor. Can you check the worst case operating temperature? If it's close to 80C you might want to cool it better, even a few degrees can have a big impact on component life.

A second possibility I see looking at your scope plot is that the supply line's inductance is causing weird transients on the inputs. You might want to try introducing the largest decoupling capacitor you can find (rated for 100V) to your system at the DC-DC converter inputs so that it can supply power as the converters switch open and shut (before current is able to start flowing on the long wire). This decoupling capacitor will also store energy as current continues to flow after it's no longer needed.