Root canal lasted 31 years. Updated to a crown a year ago. Now tooth under crown has decay. What are odds decay was resorption vs. Bacteria introduced?

Hard to say. Get more than one opinion if in doubt. The crown may have been leaking for a long time without evidence, since x-rays normally cannot penetrate the crown area if there is metal in the crown.
Odds? These types of questions are best answered by a dentist who can personally examine you, going over both clinical records and radiographs over the entire course of treatment.
Low. resorption is not common! Decay is. When crowns are replaced, existing fillings under the crown should be removed, area underneath examined for decay and removed,buildup performed, and THEN new crown replaced! Examination by an Endodontist will give you a definitive answer of decay( probable) vs Resorption.
How lucky. A root canal treated tooth lasting 31 years is remarkable. Not being able to examine you, or review the x-rays taken over those 31 years, the decay is in all probability bacterial in origin.
See your dentist. Your crown is only a year old. If the new decay ( where we always blame bacteria but poor diet and hygiene are also contributory factors) is incipient, your dentist can just fill it without anaesthetic . If the decay is beyond the gumline, the laser can remove a part of the tissue to expose the affected area so it can be restorable. See your dentist regularly. .