You are right there are lots of possible causes. Doing some research on the web will send you off the deep end with descriptions of various bacterial and viral diseases.

Young chicks are given medicated feed to address early mortality causes of some common bacterial and parasite problems that can overwhelm their young immune systems. With the age you indicate they probably are or just beginning to lay eggs (which is why you got them, right?). If there is any problem in their reproductive system cannot successfully pass eggs now would be the time that would occur. Those of us with female companion parrots handle our birds with the thought of not providing some forms of physical touching that can encourage ovulation to avoid the physical taxing of their systems and the dangerous health problems that can result with egg binding. Chickens are obviously bred for this ability to lay eggs (and the reason why most people keep them) so this is rarely a problem.

People working with chickens expect a minor amount of early mortality in their flocks and losing one is often not a point of major concern as long as you note the continued good health of your remaining birds.

If the chickens were not raised together sometimes the stranger individuals can be picked on or simply excluded from food sources by the majority. If you suspect that may be happening provide multiple food sources so everyone has a chance to eat their fill.

Be sure to take care of the carcass appropriately and remove it from the setting right away. I would suggest contacting the business or breeder you bought the chickens from and informing them. They may have no overwhelming concern, may have suggestions for you, may offer to replace the bird, may want to see the deceased bird.
To preserve a dead bird for later examination refrigerate DO NOT FREEZE. Freezing alters internal tissues and complicates diagnostic work.

By the way, in most municipalities you can dispose of the carcass of animals in your normal garbage as long as it is under 15 pounds of weight.

There are some great information websites out there to support owners of backyard chickens, of note is this one of that very name:
http://www.backyardchickens.com/

I am forwarding your question and my response on to a club member that is both experienced with raising home chickens and has good contacts with many more chicken knowledgeable folks. If any important corrections or additions need to be made I will post them in a comment here (may take a few days).