My bets, if this is is not sectarian:
1. Some sort of internecine, potentially financial, battle among certain Baptists in this state
2. Pyro
If it is sectarian:
1. Muslims
2. Muslims
3. Muslims
The "offensive" Danish cartoon.
Historically, Baptists and Protestants have settled their differences, theological or otherwise, by splitting and establishing a new church. Furthermore, all Baptist churches, from the most liberal Northern or black congregations to the most separatist, KJV only fundamentalist, are congregational in government. The Southern Baptist Convention or any other Baptist association hold no power over the individual congregations save for the ability to expel a particular church from the association. All monies are spent by the Board of Deacons or elders and all property is held by the congregation.
The area where the bombings took place likely has very few Muslims, and those that are there are probably business owners or medical practitioners, hardly the sort to be advocates of jihad. I would think that if Muslim radicals attacked churches, such activity would occur in places like the Detroit area, where they are numerous.
Unfortunately, a corruption exists in American culture, even in rural areas of Alabama, fueled by the degenerate mainstream culture and the ubiquity of narcotic drugs like methamphetamines. It is probable that the attackers are young men like Klebold and Harris, who have become immersed in heavy metal, Gothic culture, and drugs. Even the most rural communities are plagued with such misfits.