I can imagine what they might be, but how and where do they form 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 .
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)