Why are these happening (on this piece by Paul Briggs)? It is not completely clear. The glaze has plenty of carbonates, including copper, enough for over 20% LOI. But these normally produce high populations of small blisters, this is the opposite. The melt appears to have enough surface tension that the bubbles survive and endure top-temperature-soaking. And they don't pop until the temperature has dropped so far that insufficient melt-mobility remains to heal them. The glaze has an unconventionally low SiO2 content, that makes it flow vigorously, well enough that the melt is moving and collecting in surface contours. The glaze recipe is quite unconventional, any effort to "improve" its adherence to limits would likely lose the visual aesthetic. A drop-and-hold firing schedule is likely the key to alleviating this.
Blistering is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production. And be difficult to solve.
Ceramic glazes melt and flow according to their chemistry and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them.
Cone 6 Drop-and-Soak Firing Schedule