A term from the game Sorcerer?: "The Kicker is an event or realization that your character has experienced just before play begins. It catalyzes him or her into action of some sort." See also Bangs.

References:

• Rethinking the Kicker
• Using Kickers & Bangs
• What makes a good Kicker?
• Odd Kicker Question
• The Hell is a 'Kicker'?
• Prescribing the kicker