Not proven to have been caused by lightning, but a reasonable assumption; the effects are a bit more dramatic (and less beautiful) than Lichtenberg figures (cow photo credit Cairns Post)
I did some more searching and found this (link now dead) -
When lightning hits the ground, current flows through the earth in a wide area around the point of impact. This is how a lightning strike can kill a field full of cows - the long wheelbase of the average cow means that a nearby strike induces a significant potential difference across the ground spanned by the cow's front and back legs; current flows through cow, cow dies.There is extensive discussion of mammal deaths by lightning at Tetrapod Zoology, including photos of large groups or entire herds of animals being killed at one time; in such cases ground conduction, not direct strikes, has to be responsible.
That's why it's recommended that humans caught in the open should not only crouch low, but keep their feet close together - if you're not struck directly, you don't want your feet far enough apart to create a potential difference for voltage moving through the ground.
Addendum: via Laughing Squid, this infographic illustrating the points above.
(Reposted from 2009)