What is a common cause of 5th metatarsal fracture in a trail runner?

If you're a trail runner or a marathon runner or an ultra-marathoner, well, the last thing you want to do is get a fracture. 
 
If you break something, you have to sit still until it heals, and that can take a long time. 
 
What is a common cause of fracture in a trail runner? 
 
Well, that's what we're talking about today on the Doc On The Run Podcast.