Short answers, pulled from the story.
The Noakhali riots began on the 10th of October 1946 and continued through November 1946, roughly a year before India's independence from British rule. The initial massacres lasted about a week, but persecution of the Hindu population continued well into 1947.
The immediate trigger was a rumor that a Hindu monk had threatened to offer Muslim blood at a religious ceremony, which prompted Gulam Sarwar Hossain to call a public meeting and urge retaliation. A longer-term cause was a campaign of communal propaganda led by Gholam Sarwar Husseini following the Direct Action Day riots in Kolkata, which included provocative speeches, boycotts of Hindu businesses, and escalating acts of violence beginning in August 1946.
Bengal's prime minister Huseyn Shaheed Suhrawardy acknowledged 9,895 cases of forcible conversion in Tipperah alone. Investigator Edward Skinner Simpson reported 22,550 cases of forcible conversion across just three police station areas of Tipperah. The figure for Noakhali district ran into thousands and was never precisely established.
Gandhi camped in Noakhali for four months, arriving on the 6th of November 1946. Over seven weeks he walked barefoot, covering 116 miles across 47 villages, organising prayer meetings and seeking the confidence of Muslim leaders. His stay was resented by the Muslim leadership and his route was deliberately dirtied; he departed in early March 1947.
No troops were sent to the affected areas until the 14th of October, four days after violence began. The Government of Bengal had already passed an ordinance on the 29th of September 1946 prohibiting the press from reporting on communal disturbances. An investigative report by retired judge Edward Skinner Simpson was subsequently covered up, and a summary reached the public only through a leak to The Statesman.
Survivors fled in two phases: first to Kolkata after the initial massacres, and again after March 1947 when the Congress agreed to Partition and relief camps were abandoned. Around 50,000 Hindu refugees were relocated to Guwahati in Assam. The East Pakistan government's East Bengal State Acquisitions and Tenancy Act of 1950 seized the land of those who had fled, eliminating the possibility of return.