[spoiler:76dfc98c6e]I think because it happened so sudden no one knows yet what the exact cause was, hence why the NCR thinks it was some kind bomb used by the Legion.
When you explore Camp Searchlight and find the dead Legionnaires and the note on their body you discover the true cause, as well as a way of wiping out the Legion at Cottonwood Cove.[/spoiler:76dfc98c6e]
When you explore Camp Searchlight and find the dead Legionnaires and the note on their body you discover the true cause, as well as a way of wiping out the Legion at Cottonwood Cove.[/spoiler:76dfc98c6e]