Rob says that it is an avulsion fracture.
So it's not a stress fracture from running too many miles.
So he either violently stubbed his toe or he kicked something. And if there was tissue damage looking like cellulitis, he had an out.
Since it just says fracture on the photo and no other description of the fracture, I'm not going to take Rob/Dustin's word on what type of fracture it is. They have no credibility.
The most likely scenario is that Rob broke a toe while kicking Dustin's ass for making him run so much.