Easier than you think.
I broke the 5th metatarsal on both feet at the same time. All it takes is landing on it wrong. If the fifth one (generally breaks off that little nub that sticks out on the side of your foot) goes and too much stress is placed on it again the next two could conceivably pop fairly easily. If he was back pedaling quickly he could have run 10-20 yards before he recognized the issue. At first it just feels uncomfortable and then it gets worse over time.