What causes swelling of fractured 5th metatarsal?

Injury. The energy that fractured the bone also injures the tissue around the bone. The first process of repair in the body is inflammation and fractures also bleed form the bone. Both of these processes result in swelling at the site of injury. Swelling should diminish with time.
Metatarsal fracture. The swelling is usually due to bleeding from the fracture site and an inflammatory response to the injury.
Jonee fracture. According tot the x-ray, there was some dispacement(movement) of the fracture fragment. This can cause increased inflamation and swelling. The key is immobilization of the fracture. Stop the movement. See a foot specialist to determine whether a cast or possible pin/screw intervention may be necessary.