Q.Weeping cherry tree
I just bought a weeping cherry tree. Have been watering it every day. All the leaves are falling off. Why?

Watering daily is crucial for new tree plantings.
Make sure the soil is adequately draining. Make sure the new tree was planted not to deeply.
Here is a couple of links to help you.
https://www.gardeningknowhow.com/ornamental/trees/tgen/watering-newly-planted-tree.htm
https://www.gardeningknowhow.com/ornamental/trees/weeping-cherry/weeping-cherry-growing-tips.htm