I am afraid your tree is suffering from root rot. This happens to newly planted magnolias that are planted even an inch too deep or that have a heavy mulch over their roots and that are watered too much. It shows up a few months after planting or during warm weather when the rot spreads. To save your tree dig it up now. Replant in a container of potting soil either in a large pot or on a mound with good drainage. The soil level should be just barely above where the roots flare out from the trunk. Keep out of the hot sun and water when the soil is dry two inches down when you poke your finer near the trunk. If all the leaves fall do not give up. The tree needs to grow new roots then new leaves may sprout. Do not cover magnolias with deep mulch or pile mulch around the trunk. Do not overwater and always plant in soil with good drainage so on a mound never in a low spot. I think you will save it. Keep growing, Marianne Binetti