Dogs howl at sirens because it sounds similar to another dog howling, and dogs have an instinctive desire to howl together. The sustained, high-pitch sound of emergency sirens mimics a canine howl, which typically also has a higher pitch and is longer than the typical bark.
Wild canines such as wolves and coyotes recognize each of their pack mates' howls, which help them to keep in contact while hunting and to recognize intruders in their territory. Not all dogs howl in response to high-pitched sounds, but some retain the instinct. Dogs that howl at sirens may also howl at other high-pitched noises, such as musical instruments or a human soprano singing.