A quick look at the energy in a lightning stroke: [Link: en.wikipedia.org]

With this much energy available, trenching is pretty easy. On the leading edge the copper is vaporized, and the vapor creates a large, hot, conductive path that will boil off the water, creating a steam explosion that vaguely follows the easier conductive path. Mega Volts and Kilo Amps are available, we don't care about a little resistance. Our beefy 300 Amp service is puny in comparison. Even our welders can only create a short arc and heat a point.

Another way of looking at this is that there has been a trainload of electrons thrown at a point and that inertial energy can't be absorbed at a point.

From the photo I guess that the earth was relatively dry, minimizing leakage beside the conductive path. (hence the longer path)

I think that this is a relatively rare event -- requiring a high energy strike and a nearby victim conductive path. Otherwise, we would all have one of these in our backyard. I saw some accounts of similar trenches following tree roots.

It would be interesting to have a high speed, stop action video of this trench building.