Slavery was the primary motivation (just as it was the primary motivation for the 1861 pro-slavery rebellion of the US South). The weakness of the Mexican central state and its logistical difficulties sustaining its army was a convenient (for the white Texans) bonus that made their pro-slavery revolt possible.
If you want to keep digging, you can add American expansionism and ambitions to conquer big parts of Mexico, i.e. a "manifest destiny" land grab. The US was happy to tacitly support the Texans because whether they failed or succeeded it was relatively low risk for the US government and their success was likely to further US interests.
It's fair to say that the primary cause of Texan separatism was slavery.
It's also fair to say that Santa Anna was a proto-fascist and that was a legitimate factor, not a convenience. Lorenzo de Zavala, the former Mexican finance minister, didn't become Vice President of Texas for slavery or American expansionism.
Mexican elites were a brutal and repressive bunch, both before and long after this time, and "fascism" doesn't even begin to describe the evil racist systems of repression of indigenous people from the 16th up through at least the mid 20th century.
But Texas anglos didn't really give a shit about any of that. They just wanted to left alone to build their own little profitable slave plantation economy modeled after the US southern states.
Texas anglos weren't the only people involved in the Texan revolution. And while it's true that Mexican elites were largely brutal and repressive, they didn't generally generate rebellions in half the Mexican states simultaneously. Santa Anna did.
The periods before and after Santa Anna were also very violent and politically unstable, with armed rebellions all over Mexico. Santa Anna was fairly ordinary in his goals and methods by the standards of Mexican/Latin American caudillos.
The fundamental problem was not one or another particular person, but a society organized along quasi-feudal lines with extreme wealth/power concentration and systematic repression by elite landowners and a central state with limited power or legitimacy and huge logistical challenges. If it had been someone else other than Santa Anna in charge of Mexico, Texas anglo slave owners would have still had the same motivations and more or less the same systemic context, as would the US government, and eventual outcomes would have likely been similar.
If you want to keep digging, you can add American expansionism and ambitions to conquer big parts of Mexico, i.e. a "manifest destiny" land grab. The US was happy to tacitly support the Texans because whether they failed or succeeded it was relatively low risk for the US government and their success was likely to further US interests.