We don't really know. It's a scientifically unexplained phenomenon. But it tends to affect electronic equipment much more because of the use of tiny conductive bits packed very closely together. Putting lead in solder made the problem pretty much go away (in terms of whiskers growing from the solder itself, not whiskers growing on the bottom of raised floor tiles and getting blown around by the ventilation system and sucked into computer power supplies,) but since the ban on lead in solder, whiskers are back (in solder.)
Metal whiskering is a crystalline metallurgical phenomenon involving
the spontaneous growth of tiny, filiform hairs from a metallic
surface. The effect is primarily seen on elemental metals but also
occurs with alloys.
The mechanism behind metal whisker growth is not well understood, but
seems to be encouraged by compressive mechanical stresses including:
- residual stresses caused by electroplating,
- mechanically-induced stresses,
- stresses induced by diffusion of different metals, and
- thermally-induced stresses.
Metal whiskers differ from metallic dendrites in several respects;
dendrites are fern-shaped, and grow across the surface of the metal,
while metal whiskers are hair-like and project at a right angle to the
surface. Dendrite growth requires moisture capable of dissolving the
metal into a solution of metal ions which are then redistributed by
electromigration in the presence of an electromagnetic field. While
the precise mechanism for whisker formation remains unknown, it is
known that whisker formation does not require either dissolution of
the metal or the presence of an electromagnetic field.
That last sentence "does not require ... an electromagnetic field" hints to me that it is not exclusive to electronics. But with electronics you introduce the whole potential "fire hazard" and "damage to the electronics" thing.
Whiskers can affect many different kinds of metals and alloys, not just zinc. That guy who says "nothing with zinc on it is allowed in my datacenter," even if he does have an ideological point, might have a difficult time keeping tenants in his datacenter.
NASA has a whole page dedicated to whiskers: http://nepp.nasa.gov/whisker/
Here's a really informative video about specifically zinc whiskers and datacenters: http://www.dataclean.com/webcasts/Zinc-Whiskers-Webinar.wmv