In "To Kill a Mockingbird" Arthur's father had more or less locked him up in the house for 15 years. What had Arthur done to deserve this punishment?
The initial event was that Arthur and his buds were just goofing off downtown, and ended up locking the town's beadle (law-man) in the courthouse outhouse. Well, they were caught and brought before the judge, who sentenced them to
"the state industrial school, where boys were sometimes sent for no other reason than to provide them with food and decent shelter."
However, Mr. Radley thought that was a huge disgrace, and told the judge that if he released Arthur (Boo), then he "would see to it that Arthur gave no further trouble." And, he did, because "Mr. Radley's boy was not seen again for fifteen years." Then, after the alleged scissors-stabbing incident where Boo supposedly stabbed Mr. Radley with scissors in the leg, they locked him in the courthouse basement for a while until he went back to being a "malevolent ghost" in the Radley house.
Originally, there was an adolescent incident where Boo and a bunch of his friends locked the town law enforcer in the outhouse down by the courthouse. The case was brought before the judge, and Boo wound up getting sent to what equates to a reform school.
After his release, community rumors began swirling about how Boo had stabbed his father in the leg with a pair of scissors while he (Arthur/Boo) was clipping up a newspaper. The truth of the story isn't exactly verifiable, as the report had come mostly from the town gossip, Miss Stephanie Crawford. Boo had been resigned to the inside of the house after the outhouse incident and his release from the reform school, by order of his father, Nathan, the man who sealed up the tree's knot-hole to prevent Boo from communicating with the outside world.