Here is a possible explanation. They thought that the stuff put in the sphere would act like black powder but it acted like C4.
Deflagration to detonation transition is the technical term. Confinement is one way to make that happen.
The home-made devices in the photos were cardboard tubes with cannon fuses. Typical fill for these is flash powder, mixtures of Potassium Chlorate or Potassium Perchlorate or Potassium Nitrate and fine Aluminum powder and maybe a small amount of a catalyst.
These mixes are classified by ATF as high(!) explosives after some spectacular industrial accidents where deflagration transitioned to detonation and caused damage similar to the results that high explosive would have yielded. I also found this on an older bulletin board:
"Fireworks" by Dr. T. Shimizu (1981) cited an experiment in Japan wherein a common flash comp (KCLO4 x%, Alx%, Sb2S3
x% ...the common Japanese formula, what Shimizu calls his loudest) was initiated with a common #6 fuse-cap. It did indeed detonate @3100mps.
I do not think that anyone would have considered detonating 10 pounds of HE in that sphere to be a wise idea. But 10 pounds of unknown stuff that may detonate instead of deflagrate is somehow OK? If they decided to dispose an unknown substance locally because they were too worried about the risk of transporting it, then maybe it would have been be a good idea to treat the stuff with the same respect as the overaged, sweating dynamite in that old movie.
On the other end of the competence spectrum are EOD specialists who can reliably deflagrate the high explosive content of a 2000 (!) pound WWII bomb in the middle of a British town as shown
here. Despite what the title and the commentator says, this was NOT a detonation as evidenced by the lack of the characteristic shock wave and the survival of the surrounding city blocks.
