Synonyms for Instigation:

n.

incitation, goad, trigger, cause. act (noun)
fomentation.
instigation (noun)
abetment, fomentation.
motivation (noun)
impulse, motivation, provocation, incentive, stimulus, drive, impetus, prod, inducement, stimulation, lure, energy, incitement.
motive (noun)
incitement.
prompting (noun)
incentive, urging, influence, abetment, incitement, stimulation, encouragement.

Other synonyms:

cause, incitation. goad. trigger. Other relevant words:
encouragement, incitation, fomentation, abetment, urging, goad, cause, influence, trigger.