Support strut probably caused failure
Musk says a support strut holding one of the helium tanks likely fractured near a bolt attach point, and wanted to move to the top of the Falcon 9’s second stage. Several helium tanks, each pressurized to about 5,500 pounds per square inch, are mounted inside the rocket’s second stage liquid oxygen tank. The helium is routed through the second stage’s Merlin engine, where the helium warms up and injected into the rocket’s propellant tanks to pressurize the stage as the launcher burns fuel, keeping the tanks structurally sound. Musk:
It may seem sort of counterintuitive that, as the rocket’s accelerating, that something immersed in the tank would actually want to go up more, but that’s basically what happened. The buoyancy increases proportionate to the G-loading. At approximately 3.2 Gs, this strut holding down one of the helium bottles appears to have snapped, and as a result, releasing a lot of helium into the upper stage oxygen tank and causing an over-pressure event quite quickly…Within the course of a second, this caused enough helium to be released, we believe, to over-pressurize the liquid oxygen tank in the upper stage. You don’t really need to release a lot of helium because there’s only about 2 percent gaseous volume in the stage because the upper stage propellant is not being consumed.