I have a box of brown cane sugar. I pretty much only use it occasionally, in oatmeal.
It has transformed into a solid mass of sugar. A really solid mass. Like, ‘use a steak knife to stab into chunks so I can use some’ solid mass.
Obviously, this is not ideal. Is there any secret trick which will allow me to at least soften it a bit?
You can try putting it into a sealed plastic container (Like Tupperware) with a moist paper towel.
I thought sugar solidified when they absorbed moisture, not lost it? I seem to recall hearing that a microwave was a good way to soften sugar. (Make sure the box doesn’t have metal parts.)
Put a piece of bread in with it.
Put it in the microwave for 15 seconds at a time until it is soft enough to break apart. Then store with something moist, as above.
In addition to the remedies above, you can also buy brown sugar softeners – they work better than bread, apple pieces, etc. because they won’t go bad.
Brown Sugar Pillow
Brown Sugar Bear
In addition, you can prevent this from happening in the future by transferring it from the box to a plastic ziploc. Press out all the possible air and seal well. If you have one of those hand-vac devices, so much the better.
I’ve tried sealing brown sugar in ziploc plastic bag many times and it has still turned into a rock. One of these days I’ll have to try one of the damp methods (apple, wet paper towel, etc.).
Up to now if I had a recipe that I needed to stir the brown sugar into I just bought fresh (and threw out the old rocks if I had too many). If it was a recipe where the brown sugar needed to dissolve (e.g. butterscotch), then I just beat the s—t out of it with a hammer until I got it broken down enough to put into a measuring cup. Good way to work out your frustrations! 
I’ve found the fridge to be a good place for soft brown sugar, since the humidity is much higher than in your average kitchen cupboard. A tablespoon of water sprinkled on top of the sugar, taping the bag shut, and into the fridge overnight usually works for me.
Wet paper towel draped over top works for me. I learned from experience that you should always transfer brown sugar immediately to a different container upon taking it home.
Yeah, what causes this exactly?