Those class D extinguishers probably had graphite-based powders in them.
I don't know what was in them. But, the year before I started there, they had a shavings fire in a collection bin out back, and thankfully away from the building. Someone had idiotically dumped some pallets in the same bin, and those pallets ignited which of course ignited the metal. I guess it burned for hours out of control. The next week, they had those extinguishers everywhere.