I'm not saying this guy you knew was a commitment phobe, he might be a normal guy who for whatever reason lost interest.
The type of guy I'm talking about doesn't really see himself as a player in fact, he sincerely likes you which it turns out is part of his problem, it gives him the cold willies when it starts looking like the relationship is getting serious. It's really a tough situation for the guy because he genuinely wants to be close and he loves the warm feelings he has with the girl but there inevitably comes a time when he becomes so uncomfortable that he has to back out in some way, this gives him some temorary relief but it also makes him miserable because he doesn't enjoy hurting your feelings and he knows he's blown another opportunity at a real relationship. This guy is really in a bind. When the relationship starts getting serious most of these guys will start finding ways to put a distance between them and their girlfriends. They start finding fault with the woman which is very bewildering for her. They need to spend more time at work, with their friends, etc., they quit calling, they start to lie about where they are, get non committal about when they'll be seeing you again. A lot of jerks will act this way too but they aren't necessarily phobic, nice guys will usually tell you it's not working for them and they are concerned about your feelings. Phobics will sometimes just disappear temporarily or even permanently. Some phobics are strange in other ways, they won't give you their home phone, they don't invite you over, they don't want to be seen with you in public places, they don't give you information about their families. And yet, one on one they are wonderful with you. Strange stuff like that.