A picture of the tree, close up of the leaves, plus one of the whole area including the tree would help a lot.
Did the tree stay wet for days and days during the heavy rains and flooding? If it's in a low spot, or the soil settled in that area from the wet weather, it could have been wet for a while even after the flood water was gone.
It could have some root rot going on. The roots basically drown without some air in the soil i.e. drying out between heavy waterings.
