Lack of Structure/New Responsibilities
While students chafe against the structure they needed to operate within while they were attending school, the fact is that it's familiar. A lot of decisions, both large and day-to-day, are taken out of their hands. Sure, it's liberating to make all your own decisions when you graduate, but it can also be scary. All of a sudden you have to decide where to work, where to live, how you're going to get to work, what time to get up in the morning, etc. Everything from the little decisions to the big ones, and it's all on you. Again, liberating - but also enough to send put some people into a catatonic state.
Anyone can be blue for a few weeks after a major life change. But if the normal adjustment period after graduation finds you slipping into clinical depression, you need to stop it in its tracks. Go to your family doctor or health clinic, get diagnosed and get treatment.