Pyroclastic density currents interpretation
The name of the cavities are lithophysae. They show that the current has high volatile content. Vapor phase devitrification is the origin of litophysae. Pyroclasts in the curremt maintain their volatile content till compaction. volatiles escapes from the clasts during deposition to the weakness zones of the flowage. After exsolution, the laminae (source of the volatile) will be compacted and vapor phase minerals tridymite, crystobalite and feldpspar crystallized. 

The cavities don't have to be on the top of the pyroclastic density current. They may occur on the flowage shearing surfaces and a syn-depositional incident. 

The answer is not complete but it may give some idea.