Rather than normal spherical bubbles, here we have curtains of air within the ice. My interpretation of how they form is that melting occurs along boundaries between ice crystals and then the water is able to flow out once the melted areas grow large enough to intersect each other and form a continuous network. The ice would have to be reasonably close to the melting point for this to happen, otherwise the water couldn't flow very far before refreezing. The melting point at the grain boundaries can be lower than 0 degrees C because of impurities (i.e. salts) that tend to become concentrated at the boundaries.