Why did my root beer hard candy turn to a dry crumble?
I am pretty sure that this method cannot work. 

When you heat up sugar with water, you make a supersaturated solution. At room temperature, you can dissolve at most 200 g of sugar in 100 g of water, which gives you a 67% sugar solution. At hard crack stage, you achieve a 99% sugar solution. If you manage to cool that down without too much mechanical disturbance, you end up with a clear hard candy. 

But if you mix in a small amount of new, cold (room temperature) liquid, you are suddenly cooling your sensitive supersaturated solution, and dissolving parts of it. It loses its state and the sugar precipitates out, giving the crystals you described as "crumble". 

You will have to add your flavoring before the heating stage and see if that turns out OK. But you cannot add watery liquid to hard crack, unless you are adding enough to dissolve the sugar, the way people do when adding cream to caramel to make sauce.