Most rockets are fuelled by liquid oxygen (LOX) which is held at its boiling point. This makes it easy to remove the excess gas and top up the tanks with more LOX as the rocket slowly warms up on the pad prior to launch.
SpaceX's system involves super-chilling the LOX to increase its density (and therefore increase the performance of the rocket). This allowed the circumstances under which the SOX could form in the COPV overwrap, as contact of the already-chilled LOX with the much colder helium (stored inside the LOX tanks) would cause it to freeze.
Fortunately this doesn't ground SpaceX launches since the fuel loading system which caused the flaw was a new one; the company can revert to the old method and launch just fine. I'm certain we'll see SpaceX continue to make optimisations to its launch operations in the future - and this is just more proof if proof were needed that space is hard.
In layman’s terms: COPVs are used to store chilled helium that maintains the tank pressure. The investigation team concluded that buckles in one of the liners led to super chilled LOX pooling underneath the carbon overwrap. Subsequently, this liquid oxygen became trapped when pressurized, in turn breaking fibers or creating friction that ignited the oxygen. “In addition, investigators determined that the loading temperature of the helium was cold enough to create solid oxygen (SOX), which exacerbates the possibility of oxygen becoming trapped as well as the likelihood of friction ignition,” said SpaceX. “The investigation team identified several credible causes for the COPV failure, all of which involve accumulation of super chilled LOX or SOX in buckles under the overwrap. The corrective actions address all credible causes and focus on changes which avoid the conditions that led to these credible causes.”