Did your sand bed get disturbed down deep? I'm not sure if I'm on the right track , and I'm sure the real pros will correct my attempt here but it sounds like the anerobic layer of the sand bed was disturbed. Have you checked all your params? And are you sure its not the water itself? Failing anode rods in water heaters will cause water to smell sulfery...JMHO
If you get enough anarobic area, and enough build up of hydrogen sulfide gas, you can get a nice sulfur smell. ( note this gas is dangerous in close confines)
Is your sand black?
This may or may not be your problem. Just tossing it out.
Whatever is causing it, it isn't good. Sulphur is poisonous to your livestock.
Is it a deep sand bed in the 'fuge? If so, it's possible that you have some unknown sandsifting critters in there that are disturbing the layers. Try and take a closer look and let us know what you find. Good Luck!