What is happening in my loft?

We moved into our 35 year old bungalow 25 years ago. The previous owner had boarded the loft. About 10-15 years ago we laid a carpet over the boards. about six months ago I felt that there was a 6" square area that seemed to give a little. Recently that has expanded to about 18" square and I think I can detect another "soft" area somewhere else. I moved some of the clutter and rolled back the carpet so that I could access the bigger area and it seems that the fiberboard (or whatever it is) is a bit crumbly in that area and a fine dust (like sawdust) has dropped onto the insulation below. Any ideas?