The one common denominator that i notice in many of these peculiar disappearances, is that the person seems to have some change in their routine, or about to have a change.
Things like starting a new job, school, moving, or going on a trip somewhere. Maybe somebody did not want them to do these things because they will get " left behind " or maybe because they are envious. On the other hand, maybe those very changes are made by the mp. because of some dissatisfaction in their life, or, those soon to be changes, causes them stress which makes them act irrationally.
Sometimes, i think that maybe if someone is going on a trip,or getting married, just finished school, ect. they may ( perhaps uncharacteristically) want to "party" one last time before the big change. Maybe experiment with using/buying drugs or alcohol for the first time, but a "good " time could turn into a bad one quickly.
Changes often mean encountering/ mixing with new people, very exciting and/or stressful and sometimes even dangerous, for a young person accustomed to the same old high school and neighbourhood people.
speculation, imo.