Someone else mentioned job security/contracts as a possible motivation. It makes sense.

I'm less sure about the terrorist angle. The point of a terrorist act is to advance or discourage a specific political agenda; that only works if you're clear about who you are and why you did it, though. These guys never did that (although maybe scaring PG&E into securing their facilities roughly fits the definition of goal-oriented violence, even if it wasn't motivated by politics).