It was put into the kiln before it was dry (from glazing). The kiln was fired fairly fast (without using a drop-and-hold firing schedule). These glazes have significant boron, they melt early and seal the surface. But water vapor can remain until surprisingly high temperatures. And it needs to get out. So it finds a discontinuity in the glaze cover and vents and bubbles out there. That leaves these defects that even a drop-and-soak and slow-cooling did not heal.
Questions and suggestions to help you reason out the real cause of ceramic glaze blistering and bubbling problems and work out a solution
Designing a good kiln firing schedule for your ware is a very important, and often overlooked factor for obtained successful firings.
Pinholing is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production.