Lefty, I feel your pain, literally ! I noticed it about a year ago, and it comes and goes. The cracking almost feels like the cartilage is being torn apart... it sucks. I got an x ray, and nothing showed up. Apparently, when you get to a certain age (around 25), your sternoclavicular joint calcifies, turning from soft , flexible tissue to hard, crunchy stuff. If you ever injured it in your youth (I played rugby in highschool pretty sure it happened on one particular tackle, bit it didn't seem like a huge deal back then), you may have not noticed anything peculiar then, but it may be manifesting itself now. I definitely think throwing a disc is a trigger, so it keeps me from playing as often as I would like.
My doctor said that its not that uncommon, but unfortunately there isn't much you can do. Advil when it flares up, and go to the vitamin aisle and pick up some glucosamine ... it is arthritis, so taking soft tissue supplements should help.
Stretch, do some strength training, and when you do huck it back hand, take a little bit off... good luck, let me know if you find a miracle cure