The top comment from the link has a great explanation about the solid oxygen and how it may have caused the explosion.
Sorry I can't seem to copy and paste anymore on my IPad after IOS update...
Here we go:
For people who aren't understanding the "solid oxygen" comment, here's a likely scenario where this occurs:
The helium COPVs are sitting inside the oxygen tank. The oxygen tank is then filled with subcooled oxygen that's very close to oxygen's freezing temperature. The helium tanks are immersed in and covered by this liquid oxygen. The helium tanks are then begun to be filled. When you fill the tanks, initially before too much pressure builds up, the helium will be expanding from the pressure lines into the pressure vessels. This will cause an associated temperature drop from adiabatic cooling. This could cause the surface of the vessel to drop below the freezing temperature of Oxygen. There will now be tiny frozen/solid Oxygen crystals that develop in between the aluminum tank and the carbon fiber wrapping of the COPV. As the vessel is then pressurized these crystals are strongly compressed as the COPV expands by a few centimeters as it pressurizes. When you strongly compress an oxidizer and a fuel source (carbon fiber) you can cause spontaneous combustion if the pressures are high enough. This causes tank and COPV rupture and then the runaway explosion of the vehicle.
Edit: Some people don't quite understand what COPV are, so here's a nice manufacturing video of how they're made:
The Carbon Fiber is an essential portion of the strength of the tanks.
Thanks to "ergzay" for his explanation. He was then asked:
Would the tank not warm up as it's pressurized?
Yes it would, but initially the tank is empty and is possibly at a lower pressure than the oxygen tank. Thus the first bit of helium that shoots in will be expanding rather than pressurizing. Additionally if the crystals are inside the carbon fiber lattice and are being pressurized this pressure would keep the oxygen in ice form. Unlike water, ices of other materials (like oxygen) are more dense than their liquids and thus squeezing the ice would help keep it frozen.
Edit: Two other thoughts. They're not going to fill it constantly as pressurizing up to 3000+ psi (i forget exactly what pressure they use) would cause too much heating into the oxygen. They will thus likely do it in steps and let the COPV tank cool before continuing the helium filling. Then additionally, the incoming helium pressurant lines are going to be at 3000+ psi or it wouldn't be able to pressurize the tank. This means that even after the tank is already somewhat pressurized the incoming gas will be expanding into the tank.
I personally don't know how accurate ergzay's explanation is, but he seems to know what he is commenting on.
So according to EM the most likely explanation for the anomaly is a bad interaction between the supercooled liquids and the materials of the COPV. Thanks again to reddit user ergzay for the more detailed explanation of what that could possibly be.