An underground vault explosion, the second in 2 weeks, but this affected many more customers.
Off the record chatter by people in the electric utility business theorize:
1) SCE's decision to stop regular maintenance of their equipment and address infrastructure issues as the need arises. Check.
2) High demand/stress on the system (summer heat) added from people stealing electricity. In this case, it would have caused transformer explosion which affected several substations. This is plausible and not uncommon.
3) A lot of plate shifting and minor quakes going on. This coastal area may be sinking.
The Rhode Island beach explosion cause was supposedly determined:
"...scientists from The University of Rhode Island Graduate School of Oceanography have determined that it was caused by an abandoned copper cable previously used by the United States Coast Guard. The corrosion of the copper cable caused a build-up of hydrogen gas to combust"
Since media is controlled, everything has to be viewed with healthy skepticism.