When a person reacts aggressively without provocation, this behavior can be described as?

The behavior described—reacting aggressively without provocation—can be accurately characterized as truculent. This term specifically refers to a disposition that is combative, eager to fight, or defiantly aggressive. Individuals who are truculent typically exhibit a readiness to argue or be confrontational, making it an appropriate descriptor for someone who acts aggressively without any apparent reason.

In contrast, timorous refers to someone who is timid or fearful, which does not align with the aggressive behavior in question. Tetchy describes someone who is irritable or easily upset, which may suggest a shorter fuse but lacks the outright aggression of truculence. Surfeit relates to an excessive amount of something, often food or drink, and is unrelated to behavior. Therefore, truculent stands out as the most fitting word to describe the aggressive actions mentioned in the question.