It is exceedingly hard to diagnose issues without seeing it first-hand or having a conversation about it. Without any other photos or description, my two guesses would be:

1. Lightning strike on several trees in one area followed by secondary infection of Armillaria shoe-string rot, OR
2. Eutypella canker, perhaps with a secondary infection of Armillaria or some other fungus.

Might also be something like sapstreak disease. As I said...hard to tell without seeing it up close.