This is where small holes appear in the glaze when impurities in the clay aren’t burnt out during the bisque firing, causing them to escape during the glaze firing.
Try bisque firing slightly hotter; if possible, hotter than the glaze firing, to make sure all impurities are burnt out. A bisque fire to 1060C should burn out most impurities.
Use a 100mesh sieve when mixing up your glaze
Apply a thinner glaze
Leave an inch between pots when stacking the kiln