Is it not likely that those most vested in unionization, particularly those who want to take on greater union roles, are those who already recognize that their job is in a more precarious position?

If you recognize yourself as an important player in the organization, with a strong individual bargaining position, it seems likely that unionization would be seen as less pressing. Such a person may be willing to join a union in effect, but doesn't have the same kind of incentive to make a union happen.

It is certainly not surprising that the more precarious jobs are the first to go.