My hypothesis.
For Wonko - it was the No. 7 shot
For the rest of us
Tipped wads occur in factory loads
Gun Club
Winchester Universal
Dissected. Note unburned powder past the base
Enough powder remained to propel the wad partially down the barrel as the powder past the base ignited and fried the plastic. The LOUD report (NOT a squib) was because of the functionally short barrel. Short barrel shotguns ARE loud.
Jay Menefee is a good guy. Dan might alert him to this thread
http://www.polywad.com/contactus.html 
