I imagine the Ex ratted him out. The gun shop may have been investigated for selling him the ammunition, because selling it to someone with a domestic violence misdemeanor conviction or restraining order is itself a crime.
As the ex, she’d likely know about the guns and ammo at home but would she know about the new purchase? Of course the article is poorly written and doesn’t contain a lot of the relevant info needed to get the full story, but it seems to me that odd comments at the gun shop were the impetus for police action.
Thanks for helping to sort this out.