nothing to elaborate on, really. Some residency programs care about 'pedigree'. I'd wager that there'd be less stigma associated with DO school than there is with a carribean school. I don't know much about DOs in psychiatry, but that's certainly the case with regard to other specialties.
Some programs look with disdain upon anyone who didn't go to medical school at an ivy (or other equally old/monied/prestigious university). Some look askance at DOs. And others look down on carribean school graduates.
They basically think they can tell more about an applicant by where they got into medical school than what they did while they were in medical school. *shrug*