That does not appear to be buried all the deep to me.  You might also consider digging the area around the drain and putting in some pea gravel for better drainage.

Key to a frostless hydrant is its ability to drain the water in the vertical pipe when shut off into the ground below the frost line.

1. Check to make sure you're below frost line.
2. Check to make sure you have proper drainage.

Could also be a bad valve which might allow water to seep into the vertical pipe and/or drain field causing it to flood and freeze.