When bored teenagers make death threats against the president, it's terrorism... and if teenagers are systematically disrupting commerce, even just with the intent to "have lulz", that is terrorism.

>Maybe a disgruntled employee has it in for Level 3. Maybe the guy who shot up the PG&E substation has a delusional fear of electricity

Lone wolf terrorists.

>These are all plausible, non-terroristic explanations

They are plausible, but not non-terroristic. All of the things you listed are instances of terrorism.

>A legislature could decree that apple pie is terrorism, but it simply does not make it so

Since terrorism is primarily a legal concept, that would indeed make apple pies terroristic.

>We've seen no evidence that this was carried out in furtherance of a "{political, religious, ideological} {purposes, objective, cause}". We've seen no evidence to suggest that this was done to intimidate the public. Yes, it's a lousy, criminal, needlessly destructive thing to do. That doesn't make it terrorism.

This is the best argument against calling it terrorism, but I agree with the comment parent that this is probably the work of neo-luddites. California is certainly known for being a hotbed of radical ideologies and this is just a few logical steps past the bus protests. Considering it is possible to cause massive network damage by taking out a few choke points, expect these attacks to become more widespread in the coming years, probably causing a severe shortage of connectivity at times and significantly disrupting commerce.