Why was Dr. Manette arrested in the book Tale of Two Cities?

Dr. Manette was arrested for speaking out about the crimes and injustices perpetrated by the noble Evremonde brothers.

When Dr. Manette was a "young physician, originally an expert surgeon, who...(had) made a rising reputation in Paris", he was commandeered one evening by two noblemen and taken in secrecy to treat two patients.  One was a beautiful young woman with "a high fever of the brain", lying bound on a bed, and the other was her brother, a young man with a mortal wound from a sword thrust.  With his dying breath, the young man told Dr. Manette that he and his sister were peasants who had long lived under the ruthless oppression of the Evremonde brothers.  One of the brothers had become enamored by the beautiful sister and wanted her for his bed; he cruelly worked her husband to death in hopes he would influence his wife to give Evremonde her favors.  The woman's brother had come to avenge her, and was laid low by the sword of Evremonde.

Both patients died, and the brothers Evremonde, chillingly nonchalant and seemingly incapable of sympathy, swore Dr. Manette to secrecy.  The Doctor, however, was troubled, and wrote a letter to to Courts exposing the Evremondes' outrages against the peasant family.  The letter was intercepted;  Dr. Manette was abducted in the night, and put into prison for eighteen years (Book III, Chapter 10).