Why did sticklebacks sometimes display aggressively toward a red postal van?

Male sticklebacks display aggressive behavior toward a red postal van because they are responding to a specific external stimulus known as a "sign stimulus" or "releaser".1

During the breeding season, male sticklebacks develop a red coloration on their undersides and become highly territorial, aggressively defending their breeding grounds against other males. In the study of ethology, researchers discovered that this territorial aggression is a fixed action pattern—an innate, genetically determined behavior.231

The red color of a rival male acts as the sign stimulus that triggers this instinctive attack. Because the red color is the primary cue, the fish may fail to distinguish between a rival male and other objects that share that specific visual characteristic, such as a red postal van passing by their territory. The fish is not reacting to the van as a vehicle, but is instead compelled by its biology to attack any object displaying the "releaser" that signals a territorial threat.1