From the article:

"As Schlichting speculates, “The most logical explanation [is the miners] found a major discovery, and the mine owners gave them new jeans” as a reward. Outfitted in new attire, the miners may have discarded their old denim in the spot where they ate lunch or kept their tools. Alternatively, the men may have left their dirty clothes in the mine and ascended to the surface in clean gear, failing to return to the site and recover their used jeans for reasons unknown. A third explanation is the jeans belonged to the mine, which provided them to miners as a work uniform."