Now, does anyone know what it is about punky wood, especially punky knots,
that causes the same rapid and odorous reaction even in the absence of the
accelerant?

Interesting question: the commercial accelerants are (or at least used to be)
basic organic materials. So my guess is that some of the wood decomposition
products are basic enough to set off the same reaction. (I'm a chemist w/ no
forestry/botany background, note that this is a guess, not a gospel)

Kip Powers
Rogers, AR