Weeping Cherry Tree Has Brown Leaves

Two years ago in August, I planted two weeping cherry trees. The first of May we went over and the trees where fine. I went back over one month later and most of the leaves have fallen off and the few that are left are brown. We started the sprinkler system in April and increased the water time to 30 minutes every morning (before was 20). What would cause the leaves to die and drop?

Hardiness Zone: 6a

By Angela Ellis from Oronda, WA