The Southerners that settled Texas brought their slaves with them.  Mexican law required them to free the slaves, and years of legal wrangling followed.  It wasn’t the only crisis Mexico had though.  Between Indian raids in the north and south, Mexico’s military was hard pressed to maintain order.  Then Santa Anna took power and suspended the Republic entirely.  Rebellions erupted from the Yucatan to the Rio Grande, and Mexico was rent asunder.