The reference here is specifically to the succession of brutalities inflicted upon the Yaqui by the Mexican authorities.
The Yaqui had enjoyed relative autonomy under the Spanish, but the situation changed following the Mexican War of Independence when the newly formed Mexican government declared the Yaqui to be full citizens of the country and thus liable for taxation. Yaqui rebellions followed which were violently suppressed by the government forces, including a massacre in 1868 where 150 Yaqui were burned to death by the army inside a church.
Following a failed effort to win independence led by the Yaqui leader Cajemé in the 1880s, the Yaqui people were subjected to further brutality under the regime of Porfirio Díaz. Tens of thousands of Yaqui were transferred from Sonora to the Yucatán peninsula, where some were sold into slavery. Many Yaqui fled to the United States, especially Arizona, to escape this persecution.