What factors explain the rise of "Pachucos" and the subsequent 1943 L.A. riots?
I would argue that both of these things came about to some extent because of racism.
The Pachucos came into being because of racial and cultural issues. Because they were Hispanic in a white society, these youths were marginalized. Like many marginalized youths, they became what sociologists would describe as "delinquent."
When WWII came, racism was mixed with the tensions of war and with newfound mobility. Many of the sailors in Los Angeles had grown up in places without much in the way of minority populations or in which minorities were well subjugated. Coming to LA and seeing the Pachucos in their zoot suits offended their sense of what minorities' place should be in society. This, combined with tribalism (based on ethnicity and on shared membership in the military) on both sides led to a situation in which these young men came into conflict with one another in the "Zoot Suit Riots."