Tamid, the base wad does not ordinarily leave the hull upon firing. It is supposed to remain fixed in the base of the empty hull, thus the name "base wad". What is surmised that happens when a base wad causes an obstruction is that base wad is already loose, or maybe less sturdily affixed, to the inner base of the hull, and somehow the exiting payload, including the gas ejects, somehow pulls the base wad along with it for a ways until it lodges somewhere down the bore, creating a definite obstruction. What you are calling fiber base wads are really over powder wads that indeed do go down range at every shot.
Who knows how often this occurs and the base wad exits the barrel never to be noticed unless someone looked inside the empty hull? I've never loaded as many of this type hulls as I have compression formed hulls, but I've never seen a hull in this condition. I do believe it occurs, though.
SRH
