Personality disorder is the stated cause. That leaves the door wide-open, but I think it rhymes with "Not suitable for military service." And, based on the short timeline of service it wouldn't be all that different than the airman you can send home free of charge inside of 180 days. But I defer to @nittany03 and other recovering legal o's.
You rang?missed that part, back then the guys I worked with that went out with "personality disorder" were the ones that couldn't show up on time, had conflicts with others, and just didn't want to do there jobs, add in a NJP here and there, it just seemed that it was easier to get rid of people with a medical discharge due than going through the hoops to get rid of them because they were dirt bags.

I remember a couple of troops who were similar to that description; a dirtball who was too lazy to care, and a teenaged chick who just could not buy a clue if you gave her a years' pay up front. No psych shenanigans necessary. Counsel, write it down, then DRB, XOI, Mast w/ a Page 13, and Mast again. Boom. File the papers and they're out in less than two weeks. The right way. With every opportunity to stop the process in midstream by not being a moron anymore.
I can't remember if it's in the JAGMAN or somewhere else, but there's some pretty strict rules about COs ordering people to an involuntary psych eval because you think they've got a "personality disorder." And rightfully so, because they're trying to keep a shitty CO from being able to gaslight one of his/her people as a retaliation tool. Not saying psych separations aren't warranted. Off the top of my head, I can think of two nameless and rankless people I've served with who in all honesty weren't quite right up there. But I think modern society has a horribly immature attitude towards mental health. As someone who is in this business on a childhood ADD waiver, a diagnosis is a lifelong label. And so I'd be very leery of using the power of the US Navy to hang a label on one of my Sailors unless they self-refer or a doc I trust convinces me that it's needed to help them.