Could be blackmail gone unanswered (e.g pay us or we publish), could be hatred of Yahoo (either a competitioner, an employee, or just someone who feels wronged).

Or it could be a need to prove onself -- see what I can do, now fear my wrath.

Or it could simply be hatred of humanity, you embarrass people enough and they may lass out on you -- in this case by putting your password online, in other cases by bringing a gun to school.

Summing up, it strongly depends on who did it.

Yes, it really is a douchey thing to do.