Couple of thoughts.
If you always had a freezing problem then your pipe may not be deep enough, or not properly installed.
If it is a problem which is new, it sounds like the hydrant is not draining after use. Could be that dirt has filled in around the base, where stones for drainage should be, and is keeping the hydrant from draining.
Could also be that the valve is not being completely open and / or is bad, and the drain area is being flooded not allowing the unit to drain down on a timely basis after use.