[queuebert]

We didn't know as much about possible nuclear reactions back then, so I think they thought there was a possibility that there was an exothermic chain involving N or O that could be ignited by the bomb and would be self sustaining. While an asteroid impact is very powerful over large scales, it doesn't create nuclear reactions, so Trinity was indeed a first at that scale.

But, and I'm not sure how much of this they knew back then, we do get bombarded by high-energy cosmic rays, so chances are one of these hypothetical N or O reactions should've already randomly occurred at least in isolated events over the last few billion years if possible.