Actually, caffeine is a diuretic and hence pulls the water out of your system. So, in additon to causing the adenosine crash, it causes a hydrological crash as the water in your body is taken away and you are left without the added metabolic and processing benefits of the water itself -- causing you to crash, hard.

For more information on the perils of caffeine consumption, direct yourselves to the cautionary tale: Caffeinated Nasal Spray.