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Over the years I have kinda studied this problem and it does seem the whiskers are more likely to appear when a radio is left disused for long periods and much less so when in regular use.
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I'm surprised because I remember how some early computers suffered badly from this problem - and they were almost continuously powered.
FYI
Normal metals that you encounter in everyday life have very five crystal structures and are usually impure and also have been worked in some way so you may be surprised how different these metals behave when ultra-pure and single crystal. I was amazed the first time I handled a single crystal bar of pure tin...
At first it seems to have no strength at all - you expect a solid bar of metal but it is actually more like Plasticine - it cannot support its own weight and sort of flows and sags where unsupported. As it does this you see slip planes appearing as chunks of metal slide along the plane directions of the crystal. But this state doesn't last long because as it moves the crystal structure fines up and it quickly turns hard and more normal. You also notice as you handle it that it crackles - this sound is caused by a process called twinning in which single crystals suddenly switch their orientation. When the crystals are really big you can actually hear the atoms snapping into their new alignment.
It's dislocations in the crystal structure that prevents a material freely flowing and bending about. When in this pristine state the atoms can slip over each other so easily that you get this strange behaviour - and you can more readily appreciate how a fine whisker could be jacked out from a metal surface if some small cyclic force is applied to it.