Texas was controlled by the Mexican government so when Mexico called for the release of all the slaves on their territory the Americans that lived in Texas became frustrated.
Then Stephen Austin, a Texan citizen, was sent as a diplomat to Mexico City to talk about the problems and differences between the two people. He was imprisoned for eight years. This caused the Americans to start to revolt because they were not being heard and their problems were not being solved.
In a short summary, the Americans that moved from the coastal area to the Texas area became upset with the Mexican laws. So they started building up a rebellion and it turned into a big war that ended up giving the Americans the Texas Territory.