I stopped training for a couple of years and my shin couldn't handle the power of my kick, got bruised as. Conditioning doesn't last long.
All I can do is assume.
Not enough calcium in the bone? An already injured leg? Probably really tense calves that gave no give. Even overconditioning.
If you're flexible, have a decent diet and slowly condition, its not as likely to happen. But ow, poor dude!