It just seems oddly progressive for the Convention of a State that was seceding to protect White Supremacy and Slavery to include provisions that seemed to grant certain rights to the enslaved. Here is the part of the 1861 Constitution that dealt with the enslaved, which I got from here:
SLAVERY
Section 1. No slave in this State shall be emancipated by any act done to take effect in this State, or any other country.
Section 2. The humane treatment of slaves shall be secured by law.
Section 3. Laws may be enacted to prohibit the introduction into this State, of slaves who have committed high crimes in other States or territories, and to regulate or prevent the introduction of slaves into this State as merchandise.
Section 4. In the prosecution of slaves for crimes, of a higher grade than petit larceny, the General Assembly shall have no power to deprive them of an impartial trial by a petiti jury.
Section 5. Any person who shall maliciously dismember or deprive a slave of life, shall suffer such punishment as would be inflicted in case the like offense had been committed on a free white person, and on the like proof, except in case of insurrection of such slave.
Note that it says that the "humane treatment shall be secured by law", instead of may or can, so it seems that the Constitution actually required the Legislature to create the law. It also grants them a trial by a jury that's required to be impartial, and makes murdering a slave equal to murdering a white person. I looked up the constitutions of other Confederate states and they have nothing like this. In fact, many times their provisions are the opposite, such as allowing jury trials of slaves just in very specific cases. Why did Alabama, of all places, enact these provisions? Note that I'm aware that these provisions can't be called truly progressive because they still protected slavery and at most allowed a degree of paternalistic protection as the burden of the "superior race", but still, it's weird when compared with other Confederate states.
While it doesn't touch specifically on Alabama, the third comment of this long chain focuses on legal protections for enslaved persons int he antebellum period, and a growing trend to include certain, limited protections in law, which was reflective of the paternalistic self-image held by enslavers in the period and which especially gained force in the last decades prior to the Civil War, and is a trend in which Alabama's changes here fit within. Also key though, as illustrated in the linked answer, is that laws on the books did not always translate to such laws having real force, and interpretation of what they meant could very greatly.