According to gossip, why do the Radley's keep Boo at home?

Because one day in the past when he was drunk, he and the Cunningham brothers made trouble in the neighbourhood and he was arrested. Instead of being sent to the State Industrial school, Arthur Radley promised he'd keep Boo confined to the house so that he wouldn't cause any more trouble.

The town gossip suggestst that Boo is deranged and on one occasion stabbed his father in the leg with a pair of scissors. The family are embarrassed and fearful of Boo's behaviour (so the rumour goes) that they keep him locked up, but he does prowl around at night, looking in to people's windows.