It is usually caused by attendees when they've had more fun during the weekend than they usually have. This can also be caused by making friends that you can only see at cons, thus causing withdrawal and depression.
Symptoms of post-con depression may include: Lazyness, procrastination, withdrawal, whining, lack of interest, daydreaming, and soreness.
The only known remedy for post-con depression is to hang out with the friends met at the convention, or waiting for the next convention to come around. Sometimes working on cosplay or heavy planning will weaken the symptoms.