What are tin whiskers and what causes them?
Tin whiskers are filamentary growths that spontaneously grow 
  from electroplated tin surfaces.


 (from NASA article)


  The exact cause of tin whisker growth is still not fully understood.
  It is known that a whisker grows from its base and that the tin around
  the base does not thin as the whisker grows. It seems that the energy
  for growth comes from microstrains present in the tin or from
  externally applied pressure. Tin atoms appear to diffuse along screw
  dislocations within the tin and are pushed outwards by stresses.
  Growth rate varies tremendously, and it may be unsteady. Whiskers can
  fully develop in minutes or take decades to form. Spurts of growth may
  occur [1986].


Overview article: A History of Tin Whisker Theory: 1946 to 2004

There's more of references to in-depth information on whiskering here.  But, it seems that most of publications focus on prevention than on the formation mechanism.

update, related article:
Root Cause Failure Analysis of Dendrite Shorting: A Comparison
of Analytical Techniques
Printed Circuit Design and Fab magazine [it's a free-of-charge industry rag]
Dec 2014 issue, pp.28-34
(PDF or interactive viewer)