Why does a bubble of juices/fat appear on the inside of the casing/skin of boerewors?

Boerewors Information

A bubble often appears on the inside of the skin (casing) of Boerewors during braaing. Within reason, this is normal, as good boerewors always contains a reasonable amount of fat that melts as heat is applied. Some inferior fats used in boerewors have lower melting points and will bubble under the skin while the wors is still cooking. This causes the excessive, cooking pots of fat that one sometimes see. Luckily, boerewors is self regulating and the bubble usually bursts and your wors is then rid of this fat. This is however not ideal, as with those juices, goes part of the flavour.