Short answers, pulled from the story.
On the 8th of May 1963, South Vietnamese army and security forces fired on a crowd of Buddhist demonstrators in Hue, killing nine unarmed civilians and severely injuring four others. Two of the dead were children crushed beneath armored personnel carriers. The crowd had gathered to protest a government ban on flying Buddhist flags on Phat Dan, the birthday of Gautama Buddha.
Major Dang Sy, the Catholic deputy province chief and commander of the Hue garrison, was present when troops fired into the crowd. He reportedly later revealed that Archbishop Thuc, Diem's brother, had personally ordered him to fire on the Buddhists, though Sy refused to testify against Thuc. Sy was subsequently convicted and sentenced to life imprisonment.
On the 10th of May 1963, monk Thich Tri Quang proclaimed a five-point manifesto demanding freedom to fly the Buddhist flag, religious equality between Buddhists and Catholics, compensation for the victims' families, an end to arbitrary arrests, and punishment for the officials responsible for the shootings.
Diem blamed the Viet Cong for the incident, refused to accept government responsibility, and ordered the victims buried without autopsy. He called Buddhist protesters "damn fools" for demanding rights he claimed they already had. Under U.S. diplomatic pressure, he eventually agreed to a compensation package of US$7,000 for the victims' families but continued to publicly deny his government's culpability.
The Buddhist crisis was a nationwide protest movement triggered by the Hue Phat Dan shootings and the Diem government's religious discrimination against Buddhists. It spread across South Vietnam over six months, encompassing civil disobedience, demonstrations, and international publicity campaigns. The crisis contributed directly to the military coup on the 1st of November 1963 that toppled and killed Diem.
Dang Sy was tried under the government of Nguyen Khanh and sentenced to life imprisonment with an order to compensate the victims' families. Defense Minister General Tran Thien Khiem later released him, alleging the trial had been rigged to win political support. Sy subsequently emigrated to the United States.