Tilting my head until my neck cracks

I've noticed several friends and family members who regularly tilt their heads until their neck "cracks". I figure when they do it so often, they probably don't even realize how bizarre and unhealthy it looks. I've realized that I've started tilting my head until my neck cracks, and several times a day. I don't want it to become habitual. I've been travelling for over a month, so I'm wondering if it's:

• stress related?
• because I'm using a different pillow? 
• using a different chair(s) during the work day, perhaps it's posture/ergonomics? 
• something else I've not thought of?

Any suggestions on how to combat this before it becomes habitual? Perhaps my health profession or quantified self friends can provide me a process to debug and resolve?