Crazy conspiracy theory: the Chinese and/or Russians have most likely already compromised our power grid and other infrastructure control systems and are waging covert economical war by disrupting important services.
http://sanfrancisco.cbslocal.com/2017/04/21/power-outage-shu...
Possibilities I know of would mostly require failure of safety measures, but would include such things as hapless squirrels & birds, problems elsewhere in the system, and yes even remote operation of substation equipment by humans in dangerous ways. (Including both incompetence and malice, though in the latter case you would have to assume the safety measures didn't fail but were overridden.)
Addendum: I'm not coming out full force in favor of a conspiracy theory, I'm coming out full force in favor of keeping an open mind, not getting overly comfortable in one's beliefs, and remembering one knows a lot less than one thinks one knows.
From Wikipedia: "Generally substations are unattended, relying on SCADA for remote supervision and control." These SCADA systems may or may not be connected to Internet, which could allow an attacker to remotely access and modify the code that controls transformers and other electrical equipment.
In another comment, someone mentioned the power company was, in this case, pumping C02 into the substation in order to contain smoldering electrical insulation. This means that, most likely, the copper conduit heat up beyond defined tolerances. This could be due to more current being carried than those conduits are rated for. Normally, the SCADA system would be responsible for keeping these currents within tolerances. What I am saying is that they could deliver a payload to the PLCs via the SCADA that could trick the transformers into taking more load than they could handle.
Remember last time nefarious hackers infecting computers at power plants were in the news, that later turned out to be antivirus finding some spam on completely unrelated computer?
In the past 17 years I experienced 5 blackouts, so return period is about 1,241 days. Among the cities that were mentioned, San Francisco is the smallest, with population about 865,000. Let's look at all US cities with population above 500,000. There are 34 of those.
The probability of having simultaneous blackout in 3 of 34 cities during 1,241 days is 1-exp(-C(34,3)/1241^2)=0.4%, or about 2% for the same 17 year period.
The above calculation assumes independence, but in reality probability is much higher because independence assumption rarely holds in real life. For example, we already observed multiple large cities in the Northeast having simultaneous blackouts in 2003.
https://www.usatoday.com/story/news/2015/03/24/power-grid-ph...
As mentioned in another comment, Ted Koppel's book "Lights Out" is a really fascinating read on the threat.
It might not be covert economic warfare, but more of a covert demonstration whose audience is intended to be the US government. "We have lots of dangerous 0-days, and there's a lot more where these came from." In that case public credit would never be claimed.
It reminds me of how GitHub kept getting DDOSed for no reason. Turns out GitHub was a popular demo target for vendors of DDOS botnets to show their power.
Edit: could be exactly that... a demo of wares by a black market vendor of 0-days for a potential buyer.
Edit #1: a non-malicious explanation might be a vendor pushing out a bad software update to something.
I would be very surprised if there weren't at least 3 power outages in random cities on a daily basis, you just don't hear about it until it's 3 major cities.
Which includes a link to this Google spreadsheet: https://docs.google.com/spreadsheets/d/1AdxhulfM9jeqviIZihuO...
At a glance it looks like 1-2 dozen per month, so a coincidence is definitely plausible.
Have to wait and see.
I wouldn't even be surprised if it regularly were 3 major cities at about the same time and it's just that statistically someone had to end up taking notice and trying to make sense out of it when it's actually a somehow random† distribution of events, the latest of which happening to be clustered.
But digital warfare seems to somehow look more exciting to many, probably because it's intuitively less frightening than waving nukes at each other's nose, while it may very well impact people's lives for the worst in a very large, non-obvious and pernicious way.
† Some events may be random but aren't independent across because of interconnections and dispatching woes[0]. Managing a power grid or three is a fascinating thing in itself.
I don't think SF ranks in the top 3, I'm almost certain that Chicago would rank higher in economic significance, and I'd guess that Houston is above SF, probably Boston too.
Despite SF being so popular in the technology space, it's really quite a small city -- only around 800K people, compared to nearly 4M in LA, and 2.5M in Chicago.
EDIT: I found a ranking from 2010 and SF is #9
https://www.theatlantic.com/business/archive/2012/06/america...
New York LA Chicago Wash DC Houston Dallas Philadelphia Boston SF