What causes the sand in a above ground sand filter to turn hard like a rock?

I would imagine the filter has had a very poormaintenance regime and the sand has actually calcified. Calcification can be caused by Hard Water or High pH. I would recommend a better management of water chemistry and a sand change.

I would also recommend a manual inspection of the sand say every 3 months and maybe even disturb the sand manually. I would have a Word with your local pool company if you have never done a sand change as care has to be taken with the components.