The x-ray theory doesn't fit for two reasons. The defective ink would have to be x-ray fluorescent (artifacts have higher developed density). And the x-ray would have to hit the spooled film after it was shot (artifacts align in post-consumer position).
Reminds me I have a theory to test. I believe it's environmental moisture absorbed into the emulsion when the film is in camera and which softens the emulsion slightly. Then when film is wound to the take-up side, the moisture escapes through the hydrophilic background paper where emulsion re-hardens while moisture is trapped by the hydrophobic ink where the emulsion remains locally soft. Then when developed, the softer emulsion takes on developer more readily developing sooner/to a greater density than the re-hardened adjacent emulsion.
To test my theory, I reeled some film onto stainless reels and put them in a stainless tank. They've been waiting to be developed since 20-DEC-2018. I bet they have reached equilibrium by now.
If you really want to try to force a failure you can use a high humidity room. We ran several types of accelerated life testing. Common practice for plastics, machinery etc. Is high temperatures and humidity.
Temperature really isn't possible with film, we had a room that was simply called, "Florida" 80 F 100% humidity, the dew point was at 85F or as high as it could go. Water would condense on any cool surface. We were testing Amana, Maytag, Whirlpool, etc refrigerators. You could spot thermal short circuits in insulation, see where gaskets didn't seal etc.
Try putting your film in a Tupperware container with a damp sponge overnight, the gelatin will absorb moisture as well as the paper.
I am careful but I don't think I'm that careful. I'm sure Kodak and all the manufacturers do this kind of testing. I developed 2 batches, 4 rolls of 120 TMY2 in XTOL 1:1 the last 2 nights everything looks perfect, I keep temperature at 70°F through the entire process. I follow the manufacturer instructions to the letter. I never have had a problem.
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