Crazy reality here: SF was caused by a substation fire.
http://sanfrancisco.cbslocal.com/2017/04/21/power-outage-shu...
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.