Not a whole lot of data is presented in the article, but they do say that important areas of the launch site – the LOX farm, the fuel farm, the Falcon Support building – were largely unaffected by the catastrophe.
Purest speculation: a helium tank popping would of course be terribly bad, but it would not seem to cause a big sudden fireball. But a high pressure tank cutting loose, even when filled with an inert gas like helium, could easily trash tanks and lines containing LOX and kerosene, mixing the two. Again, though, simply breachignt he fuel and oxidizer in this case wouldn;t *necessarily* lead to combustion.
Some extreme handwaving, though… they were apparently using supercooled LOX. By using liquid oxygen at a temperature well below what they’d need to to keep it liquid, the exterior of the tank could in principle get cold enough to chill liquid oxygen out of the *air.* So, *maybe,* the tank would have liquid oxygen condensation on the outside, where it might mix with carbon fiber components, or cork insulation or some other carbon or hydrocarbon structure. The *possibility*exists that the oxygen could seep into those components. And anyone who has ever soaked a charcoal briquet in LOX and then shocked it (by, say, hitting it with a hammer, dropping it from a great height or popping a high pressure tank of helium next to it) knows that such a mixture of carbon and LOX is quite unstable and will detonate.