When extended periods of below freezing temperatures occur before the leaves are fully cured the mid ribs and veins of the leaf can freeze.  The liquid contents of the cell expand when they freeze causing the cell walls to burst.  When warmer temperatures return the liquid thaws and leaks into the surrounding leaf tissue resulting in irreversible staining. 

Note how the dark stains tend to follow the major veins of the leaf.  These stains generally do not lighten with time and the resulting leaf is of low quality.