Southern States had a long history of restricting the rights of freed slaves. There had always been a significant free black population in the South and the Southern States had laws to limit the freedoms of many of these Blacks including in most cased their right to vote. With the war over and all of the slaves freed Southern whites both feared retribution from the slaves while at the same time needing their labor. White southerners continued to believe despite their loss in the war in their racial superiority and in their belief that Blacks were destined to serve them.
In response the legislators of the Southern States passed new laws aimed at the freed slave. These laws limited what the newly freed slaves could do. At the heart of the laws were vagrancy rules that allowed the police to arrest people for hanging around not doing anything. These people would be arrested fined and then forced into becoming indentured servants to pay off their new debt.
Northerners were furious at these actions. To many of them they had just fought a bloody Civil War whose positive outcome other than preserving the Union was the fact that slavery had needed, this seemed an attempt to enslave the newly freed slaves by other means. The result was the passage of the Reconstruction Acts and the enforcement of those act by the army.