There are certainly a lot of puzzling aspects to this. On the one hand it clearly wasn't just a couple of yahoos looking for some fun after a night of drinking. There seems to have been quite a bit of planning involved. Also, someone clearly knew something about the technical aspects of how substations and transformers work, and where to access and cut the fiber optic cables. This suggests either inside knowledge and/or considerable research. All of that, plus the reports that over a hundred rounds were fired suggests that it was probably not a lone individual, but more likely involved several people.

On the other hand, one must ask what was the motive was? If they intended to disable the grid, they failed. One article said that this substation was fairly far down the list of most critical points in the California grid. Apparently there are other substations that if disabled would have a much bigger impact on the electric grid, and would be harder to work around. (For obvious reason the power companies aren't saying which those are.) So why go to all this effort (and risk of being caught) for a relatively low value target?

Some have suggested that it was a dress rehersal for a larger, more devasting attack. To me that also seems a bit puzzling. If one is planning a larger attack, why tip off the target about your methods? As a result of this attack the government and electric utilities will obviously take whatever small steps they can in the short term to make the infrastructure less vulnerable, and increase security at the most critical points. And in the longer term they will likely make bigger steps to harden the system. Any new substations will probably be built with more security in mind. And the FBI is now on the case. While none of this will make it impossible to do a similar, larger attack, it will become more difficult with a greater risk of getting caught.

All in all very puzzling.