There's no ignition source normally — the liquid oxygen isn't under as much pressure, it's just sitting in the tank under nominal conditions. The idea is that the solid oxygen formed underneath the top layers of the carbon overwrap. Then as the aluminum liner of the the tank (which is under the composite wrapper) expanded as the tank was filled, the solid oxygen was subjected to enormous pressure, creating the ignition source — right next to a fuel source (the carbon composite), all in a big tank full of oxidizer.