What are the psychological mechanisms for compulsive talkers? What are the psychological mechanisms for people who can't seem to help frequently "talking too much"?

Specifically, I am referring to those that recognise cues to when someone is busy, disinterested or distracted, but are compelled to continue talking.
Compulsive talking appears to be linked with the inability to manage social anxiety. Whether the anxiety is brought about from the uncomfortability of the interaction or from the avoidance of deep seated feelings within the individual.


  People who score beyond two standard deviations above the mean on the
  scale are labeled as “talkaholics,”  


as cited by James C. McCroskey &amp; Virginia P. Richmond 


  The most common and basic cause of any compulsive behavior is anxiety.
  A person engages in compulsive behavior not because he particularly
  wants to, but because he’s so anxious that he has to do something to
  get his mind off that anxiety. Until the compulsive talker comes to
  grips with whatever he or she doesn’t want to think about, the talking
  will continue.
  
  The root of this anxiety varies from individual to individual. But it
  often stems from deep-seated emotions of insecurity and
  inadequacy.../...
  
  For the compulsive talker, silence and reflection are simply
  unacceptable. A pause in conversation is an emotional abyss into which
  they simply cannot help but throw themselves.    


written by Dr Hurd

Compulsive talking could be confused with Pressure of Speech. Which can be associated with mania and attention deficit disorder. Pressured speech is marked by speaking rapidly, with a sense of urgency and, frequently, with no social awareness. 

Logorrhea is another condition, which causes the person to talk endlessly, but, usually, in an incoherent, rambling fashion.



Identifying compulsive communicators: The talkaholic scale
James C. McCroskey &amp; Virginia P. Richmond
Communication Research Reports
Volume 10, Issue 2, 1993  

Talking too much can have deep-seated roots
Dr Hurd
Private Practice of Psychotherapy and Solution-Focused Consultation since 1988.
Multiple published works.