early 15c., "something which occurs casually in connection with something else," from Middle French incident and directly from Latin incidentem (nominative incidens), present participle of incidere "happen, befall," from in- "on" + -cidere, comb. form of cadere "to fall" (see case (n.1)). Sense of "an occurrence viewed as a separate circumstance" is from mid-15c. Meaning "event that might trigger a crisis or political unrest" first attested 1913.
"conducive (to), contributing (to)," early 15c., from Middle French incident (adj.) or directly from Latin incidens, present participle of incidere (see incident (n.)).