Stories are made because something, in a moment, changes everything. This moment is the story trigger.
Until this moment, life, no matter how dysfunctional, has gone on, and the character has maintained a balance, no matter how precarious.
Though the trigger is significant, it needn’t be a literal explosion. It might be as subtle as a strand of hair found on the couch cushion.
In this moment is the detonation of consequences that your main character(s) must struggle to manage while striving to achieve a new equilibrium.
Writing prompt: beginning with one (or all) of these pictures, select a trigger to start your story. Write about what happens in that moment that ruins everything.