To voyou: Intentional
The picture is actually cropped from a larger image. Shooting fireworks, or any event, I try and ensure that the action falls within the central 60-80% of the frame. Thus, if something unexpected happens close to the edge of the photo, or even just outside, I don't loose it (this is just practical experience from loosing hands, actions, people that could contribute to the image if they were totally in the picture). The cropping here was to ensure that the critical element(the fireworks) were totally in frame.