I’m not sure that I would believe that LAC graduates who get PhDs are more likely to be perpetual adjuncts than research university graduates who get PhDs - especially given that there’s a wide range of both kinds of schools. Perpetual adjunct-hood seems to have a lot more to do with where and how you did your PhD, as well as luck (the market is just really bad in some areas - so an English or history PhD from even a well-reputed program, maybe top 30-40, could still end up adjuncting for several years just because the market is so tight and because they don’t want to leave academia altogether).