Based on the news article, the women attacked Mr. Brown's store because they found out that he spied for the Union army.
A spy clandestinely obtains military information in enemy territories. Spying is a high-risk criminal offense. If a spy is exposed, they face punishment without trial. Another name for spying is espionage.
Thus, the women attacked Mr. Brown's store because he was spying for the Union army against the Confederate army.
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