What events led up to the Buddhist monk riots referred to as one of the turning points during the Vietnam era?

The main event that led to the rioting by Buddhists
against the Diem regime was the self-immolation of a Buddhist monk named Quang Duc on a
street in Saigon.  This happened on June 11, 1963.

This
monk burned himself to death in protest of the policies of the Diem regime.  Ngo Dinh
Diem and his family were Catholic.  The vast majority of people in Vietnam were
Buddhist.  Even so, Diem discriminated against Buddhists.  An example of this came when
he did not allow Buddhists to conduct public celebrations of a major religious holiday
in the city of Hue.  This led to protests that were repressed
harshly.

In response to actions such as this, Quang Duc
killed himself.  His example, in addition to the insensitive (to say the least) reaction
of the Diem regime (Diem's wife welcomed suicides like that, using the word "barbeque")
led to huge unrest among Buddhists in Vietnam.

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