There are several theories as to why Governor Faubus sent the Arkansas National Guard to Central High School, but off the top of my head the one I remember from Arkansas history classes that seemed most likely involved the upcoming Governors race. Faubus was a progressive who had already desegregated public transportation in the state by 1957, but by the mid to late 1950 the state of Arkansas saw a dramatic rise in the number of segregationists. In an effort to attract voters and increase his popularity with the large number of voters who opposed desegregation in Arkansas, Faubus decided to send the national guard to Central High.

I found this interview with Orval Faubus from 1957 where he gives is side of the story. Basically, he tells Mike Wallace that the decision to send the national guard to Central High was made in an effort to protect the students. He says that he didn't feel that the school could have been integrated peacefully at the time. In all honesty, he may have been right about this judging by the stories and video footage of the Little Rock nine's first attempt to enter LR Central.

There are a number of documentaries about the little rock nine, and it's covered in almost every documentary on the Civil Rights Movement. A good one to check out would be "Eyes on the Prize" which you can watch online I believe. The second or third episode covers Central High I think. It's been a while since I've seen it, but it's worth watching.