Non-stop talkers don't even pause to take a breath. It's as if they fear silence. What causes that to occur in some and not in all of us?

Nerves.  Or lack of control of them.    They get manic inside and do not even realize they are running on a wheel of words.  IT DRIVES ME NUTS.  

They think that someone is going to interrupt them and their line of thought and they cannot run that risk.  They have to control the conversation and they have to talk faster and longer and longer and longer and bore everyone to death.

They create their own fear.  They are fearing being cut off and ignored and they are causing it.  By acting this way.  They are like gerbils on wheels that have no idea why they are having problems communicating.  BECAUSE THEY DON'T EVEN KNOW THEY ARE DOING THIS.