One of the lids on a quart jar of green beans "popped" after sitting on the kitchen counter for two days. It was one of a batch of a dozen jars canned in that session. The others are fine after a week. The jars were processed in a pressure canner at 14# for 25 minutes. The jar was placed in the fridge. Should the contents be discarded? Also, ideas on what could have caused this? I'm thinking a defective lid?
Yes, the contents should be discarded. Putting it in the refrigerator after the seal fails is too late.
Time and temperature kill pathogens, so it's not so much the pressure as the through-and-through temperature of the food that matters... Something insulating in the beans could harbor bacteria. I've seen a fly grow an interesting little blob of an ecosystem.
A defective lid or something on the rim causing physical air infiltration are more likely to fail by the time it reaches ambient temperature. A delayed failure isn't impossible, but there should be evidence of a defect on the seal.