Crawling is where the molten glaze withdraws into islands leaving bare clay patches.
Causes for crawling;
Presence of oil, grease and dust- reduce handling of bisque ware and use damp sponge to clean pots before glazing. If you are using underglaze powders mix with Universal Medium instead of water
Firing- initial firing stage should be slow to allow moisture and impurities to burn away, keep the kiln ventilated until 400°C then add bung.
Glazing- cracking of glaze layer during drying can cause crawling. Also, ensure the glaze layer is completely dry before firing