There are theories...
...all a bit tenuous, but based on SCIENCE!, at least for the most part.

One I came across a few years ago is to do with lightning striking sandy soil. Small, tube-like silica structures do occasionally form in such soil, and I think the idea was that the energy from the lightning strike discharges this structure from the soil. Because it's just been struck by lightning, it's very energetic and heavily ionised, hence it glows. Because it's ionised, it will seek to earth itself through conductors, such as metal forks and of course it will dissipate its energy fairly quickly.

A bit tenuous, as I say, but not a bad explanation. Another one involved microwave discharges ionising parcels of air, though I think that was an attempt to explain the fireballs occasionally (rarely) seen in the upper troposphere.
(LongJohnBaldry, Mon 7 Jul 2008, 21:59, Reply)