I think reading between the lines a little bit should indicate that because 50%+ of professors are adjuncts means that universities are proactively attempting to keep professors with that status. It means they can pay them less both in salary and in benefits. Hence why Duequesne would want to avoid unionization of it's adjuncts.

I don't know the statistics of adjuncts who have full time jobs as well, so take that with a grain of salt.