Was that caused by decomposition, or had they filled it with shavings?
Weird.
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.