As reported by the San Jose Mercury News, the "mystery" as to why the pipe ruptured appears to be solved. The NTSB found that the pipe was not properly welded, even by 1956 standards.  The welds were supposed to go through the whole pipe but didn’t.

In the area of pipe identified as the spot where the rupture occurred, the weld penetrated only about 50 percent of the pipe wall.

In other words, the pipe was at less than half strength. Though it’s not clear whether it was a PG&E employee or one of PG&E’s favorite contractors who did the defective work, it makes no difference.  If PG&E hired the work out, PG&E still should have had an inspector on site approving the welds. In short, whoever actually did the welding, the integrity of the final product was PG&E’s responsibility.Defective PG&E Welds

Right after their pipe exploded, PG&E promised:

If it is ultimately determined that we were responsible for the cause of the incident, we will take accountability.

So, is PG&E stepping up and taking accountability?  Not exactly.  PG&E has removed the old pledge from its website.  Now it is singing a different tune. While expressing "appreciation" to the NTSB for the "meticulous and painstaking work by its experts," PG&E nonetheless insists

  it’s premature for PG&E or anyone to speculate on what caused this accident. 

Huh? The NTSB’s "meticulous and painstaking work" found that PG&E’s welds were bad.  PG&E is responsible for those welds.  What more does PG&E need before owing up to its responsibility?