The handle on this mug joined the wall at an acute angle. The wet engobe was applied to the leather-hard piece and took hours to dry (rewetting and softening the whole piece in the process). During that time the engobe was able to exploit the join and penetrate into it (the crack diverted straight into the soft weakened wall). Notice how the engobe has pooled at the join in the other two mugs also (although they did not fail). Engobes typically have a high water content and experiences like this teach us to take precautions when using them.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.