It looks like excelsior to me.
I am guessing that the lining decomposed and that allowed the stuffing behind it to fall in to the bottom of the coffin.
The article said that it was a simple Pine coffin (read cheap prison supplied coffin). That means cheap lining that would decompose quickly. The article also said that the coffin was water damaged. That also means that water was also likely inside the coffin contributing to decomposition of a biodegradable lining.
I know that this is not a popular view...but I can’t understand spending thousands on any coffin. All of that expensive woodwork and COMFORTABLE lining is such a waste. IF you want a funeral showing the body, there could be some sort of elaborate couch; but burying all of that money is silly.
I’ll be cremated...and even for that you have to BUY A BOX! Insane...buy a box to burn. The cardboard box is about $70...but that’s much better than those lovely work$ of art.