Popular Science: Yesterday Musk explained to CNBC why it took so long to get to the bottom of the explosion: “It was a really surprising problem. It’s never been encountered before in the history of rocketry.” by Sarah Fecht
'The problem has to do with some super-cold oxygen reacting with the carbon fiber composites within the fuel tank.
'The Falcon 9 rocket flies by combusting liquid kerosene with oxygen. Because there's no oxygen in space, the rocket needs to bring its own. To pack in as much fuel as possible, most rockets cool oxygen gas until it's liquefied; SpaceX takes it one step further by cooling it even more, possibly to increase the density and thus how much fuel the rocket can carry.
'Well, it turns out that the supercooled oxygen might have been too cold—cold enough to actually solidify.'