Was Santa Anna a particularly oppressive leader?
What events caused Texas to declare independence from Mexico?
Were there many in Texas who remained loyal to Mexico? (Like the Tories in the American Revolution)
Slavery and religion.
As I understand it, many of the people who rebelled were immigrants to Texas/Mexico from the U.S. (called Texians). They were mostly protestant and Mexico was officially Catholic. They were also pissed off when Mexico outlawed slavery in 1829. The right to slavery is in the Constitution of the Republic of Texas, so slavery was obviously an important part of the picture.
I don't know very much about Santa Anna, but he was a centrist and disbanded the militias, which pissed of the Texians.
Wikipedia says that some Tejanos (latino Texans) worked as spies for Santa Anna, and there were loyalists. I think detailed answers to your questions can be found at the wikipedia article on the subject, which I found very interesting:
http://en.wikipedia.org/wiki/Texas_Revolution http://en.wikipedia.org/wiki/Constitution_of_the_Republic_of_Texas