Q.Nectarine trees
We just planted a nectarine tree a few weeks back it has started to loose its leaves. Why would that be

Make sure you have planted in soil that is well draining.
You will need to water daily for the first 2 weeks and then you can cut back.
Adding 3 to 4 inches of hardwood mulch will help with moisture retention.
A newly planted tree can drop leaves.
This article has more information on care needed.
https://www.gardeningknowhow.com/edible/fruits/peach/peach-tree-care.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/watering-newly-planted-tree.htm