Slavery was illegal in Mexico? Santa Anna was a dictator. All the people in Mexico were his slaves. From Wikipedia:

"...He formed a new Catholic, centralist, conservative government which replaced the 1824 constitution with the new constitutional document, entitled "The Seven Laws" (Constitution of 1836). Santa Anna dissolved the Congress and began the centralization of power. The regime became a centralized dictatorship backed by the military.

Several states openly rebelled against the changes: Coahuila y Tejas (which was to become the Republic of Texas), San Luis Potosí, Querétaro, Durango, Guanajuato, Michoacán, Yucatán, Jalisco, Nuevo León, Tamaulipas, and Zacatecas. Several of these states formed their own governments, the Republic of the Rio Grande, the Republic of Yucatan, and the Republic of Texas. (Only the Texans defeated Santa Anna and retained their independence)."

Texas rebelled because Santa Anna dissolved the constitution the state had agreed to. Santa Anna broke a contract.