We're exploring the ABC framework for understanding behavior: what comes before the behavior (the Antecedent), the Behavior itself, and then what happens afterwards (the Consequence). The Antecedent anticipates the Consequence. Within the Antecedent is the trigger: what sets off the behavior. The trigger suggests both path and destination - behavior and pay-off.
But just because a trigger happens before the payoff doesn't mean the process for understanding a behavior begins with considering the trigger. Nope - you gotta start with the pay-off, because without a possible pay-off there is no trigger. The trigger is an affordance - that is, an enticement to achieve an outcome by doing something. We turn a doorknob to open a door.
Something sets off a behavior because it is perceived as an occasion to act in a particular way to achieve a particular outcome. Someone frowns at us and we respond with a joke to lighten them up. Or look away to ease our discomfort. Or, knowing this person has a facial tic that looks like a frown, we ignore it so as not to embarrass them.
We have competing fears, desires, intentions and goals, so we have to choose what pay-off to go for. All sorts of things inform our choice of pay-off, for instance: ease of achieving, certainty of achieving, vividness, immediacy (time horizon), resolve/commitment, self-efficacy*, anticipated pleasure or relief, impatience, impulsiveness, momentum, and expectations (or not) of future opportunities for the same pay-off.
A lot of things to consider but usually these choices are made in a flash, below the threshold of consciousness, thanks to our extraordinary brains.
* In other words, confidence in our ability to pull-off the behavior to achieve the pay-off.