Trees die from many causes, more than half of which are not disease or insect pests but rather abiotic causes.  Abiotic means that there was something wrong with the environment, such as too little water or damage to the root system from construction.  We cannot make a diagnosis without more information about the situation of the tree and/or signs and symptoms. 
Please send us as much information as you can to help with the diagnosis. Also read our website publication, "Common Abiotic Plant Problems," which may pinpoint the problem.
ECN