It's the sole dead sole which normally exfoliates. But in the case of false sole it never exfoliated and just began packing itself onto the bottom of the hoof. When it begins to build up thick enough it puts pressure onto the internal structures of the hoof, causing bruising. It can get real thick too. There's some videos and pictures on the net showing just how much it can push up into the hoof causing extreme pain for the horse. I'll look for em.
Truth passes through three stages. First it is ridiculed. Second it is violently opposed. Third it is accepted as self evident.