Powder has a deterrent on the outside of that granules that gives it the grey colour. It is blown off under these conditions revealing the yellowish coloured nitrocellulose. Some of the granules clump together I think because they melt a little bit without igniting.

For the issues I had, the powder column and wad did not move at all in the case, but still the projectile was forced into the rifling (by a pressure wave), and with the yellow, clumped powder remaining.