The Wikipedia article on this is pretty decent, but essentially: Inherent social tension and economic inequality, never very far from the surface of Salvadoran life, boiled over in especially brutal ways in the 1970s. In 1979, a junta of military officers took power and enacted some moderate economic and land-holding reforms. This, however (in a pattern which was to repeat throughout the war) provoked howls of protest from the Salvadoran landholding elite, who generally saw any attempt at reform as a capitulation to Communism. The Salvadoran left, in turn, scorned the junta as a tool of the aristocracy. There were massacres and a lot of tit-for-tat warfare.

However, none of this convincingly answers the question of why full-scale war broke out in '79-80, since all the component factors (coups, peasant massacres, cursory reforms followed by reactionary corrections) had been going on for decades. The major difference this time was that the urban Salvadoran left was able to largely put aside partisan squabbling, and successfully enacted the classic Communist strategy of sending their urban cadres into the countryside to organize the campesinos, particularly in the remote province of Morazan. The united FMLN of five different leftist groups was imperfect and sometimes violently messy (infamously, the poet and ERP militant Roque Dalton was assassinated, not even by another constituent group, but by a rival faction of his own ERP).

A lot of this is just atmospheric memory, since my family was very tangentially involved, but two books which I found informative are:

López Vigil, José Ignacio. Rebel radio: the story of El Salvador's Radio Venceremos. (Translator, Mark Fried). Willimantic, CT: Curbstone Press, [1994]. Translation of Las mil y una historias de Radio Venceremos. ISBN 1-880684-21-7. Note: This is an oral history, and almost all the participants are FMLN soldiers, so it should be taken with a grain of salt as to objectivity (for example, intra-FMLN feuds and violence don't get a lot of coverage, although that's debatably due to assumed familiarity with them at the time), but it's definitely a useful source, and there are several interviews about the process by which FMLN cadres arrived in Morazan at the beginning of the war, and the motivations by which they decided to forsake peaceful activism in the cities.

Danner, Mark (2005). The Massacre at El Mozote: A Parable of the Cold War. Granta. ISBN 1862077851.