The initial event was being caught causing ruckus in the town with his buddies when they were teenagers. They were just goofing off, 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.
In To Kill a Mockingbird, Arthur "Boo" Radley ends up being locked up in his own house for fifteen years because of his father's overly strict reaction to some teenage fun and games. Boo and his friends were goofing around and locked a lawman in an outhouse at the courthouse. Unfortunately for this band of merry pranksters, they are sentenced to go to the state industrial school.
Mr. Radley intervenes and promises the judge that he will take care of the situation; this results in him isolating his son from the world for fifteen years, which severely damages Boo's social ability and reputation within the community. That reputation is only further hurt by the rumors that Boo has stabbed his father in the leg with a pair of scissors.
Ultimately, Boo spends a long sentence in the prison of his home, even if he does manage to escape jail time.