Plastics or oil based fibers , will appear to disinegrate after exposure to long periods of heat. We have all been taught that oil based fibers will last forever... it is true the molicules will be around long after we are gone. But they may not be connected to each other as originally manufactured. Many cotton battings have a poly scrim. That does not mean your quilts made from man made oil based battings will have te same result.. there needs to be a catalist , such as heat .
It possible this batting was sorted in an attic or someother place that caused the fibers to break down some.