What causes pin holes in bark of fruit tree?

A pre-existing wound or excessive pruning exacerbated by fluctuating weather and growth extremes may cause the bark to split on fruit trees. /

The bark is generally made up of dead cells, on the other hand, secondary growth in the vascular region of stem keep on increasing the girh. The cell of the secondary cortex keep on dividing and elongating to keep pace with the girth of stem. The bark cells being dead can neither elongate nor divide, hence it starts splitting.