Both long term stress and sudden environmental changes can cause the tree to exhibit the symptoms you noted. Typically poor growth in the current growing season is a result of factors from the year(s) before. You could look for borer damage on the trunk and larger limbs, check for the annual rate of growth the past several years. if you follow back the rates of growth, there could be a decline in started in 2013 due to the 2012 drought. A certified arborist upon inspection could provide more information