Not sure where you live, but there's a good chance that was damage from intense sunlight. I'd bet you that when they all died back, the affected trunks were all facing the same direction. Nothing to worry about now because they seem to be well on their way of closing the wounds.

If you live in an area with intense sunshine and heat, consider painting the trunks white with diluted latex paint.

-Tom

Heat, cold, mechanical damage. I have this on a few northern side of trunks... from freeze damage from several years ago .... (just about healed up)