I don't crack my sternum but I know people who do. For most people, that is not an area that has tension build up. It probably is a combination of your body's muscles and bones being slightly different in how they support weight while you are sitting and standing, how your chest muscles tense up if you are stressed, and possibly some specific manner of how you sit/slouch (that's a huge factor when I need to crack my back)

I'm a pro at self-cracking my back. Sometimes when college was making me super stressed, I would be able to get 10-15 seperate audible cracks. I'm glad I haven't been that stressed in a long time(in fact I haven't cracked my back probably in a month or so!)