From the article:

Despite strict fireworks bans in many cities, including Oakland, they’ve become a year-round nuisance in the Bay Area. And one of the primary ways they’re spread is through the enterprising but illegal work of small-time dealers who obtain the contraband from licensed shops outside of California, sneak it into the state, and then sell hundreds and even thousands of pounds of explosives out of homes, vehicles, storage units, and even corner stores.