There are some theories that the originally stocked birds did not have adequate genetic diversity to adequately sustain 30 or 40 generations.  Transplant those birds from LBL to other sites in TN back in the 80s and the same underlying problem remains... inadequate genetic diversity.  Same picture everywhere with LBL birds.  Massive population explosion, with complete population collapse.  There needed to be an infusion of other genetics into the flock for them to be a le to propagate indefinitely.

Is this more than just a theory?  Who knows.  But it does fit well with the pattern that those areas first restocked in TN saw populations explode followed by population collapses, while those restocked later with more diverse genetics seem to be faring much better.