provocation
An action meant to annoy someone or make them react.
A provocation is something someone does or says deliberately to make another person angry, upset, or react strongly. When your sister keeps poking you or repeating everything you say, knowing it annoys you, that's provocation. She's trying to get a reaction from you.
Provocation calls forth anger, frustration, or some other strong emotion. A bully's taunts are provocations meant to upset someone. In sports, a player might use trash talk as provocation, hoping to rattle an opponent and throw them off their game.
Sometimes provocation is obvious and mean-spirited, but not always. A teacher might use gentle provocation to spark debate in class, deliberately taking a controversial position to get students thinking and arguing. A little brother might see how many times he can say “Why?” before his older sibling gets frustrated.
The key element of provocation is that it's intentional. Someone who accidentally bumps into you isn't provoking you. But someone who keeps “accidentally” knocking your pencil off your desk is engaging in provocation. They want you to react.
When something happens “without provocation,” it means the person acted aggressively or angrily for no reason at all, which makes their behavior even worse.