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Unfortunately, your photos did not come through. I am unable to say what what was the exact cause. There are some guesses, though. Late frosts are common reasons for distress. Usually, this affects flower buds more than other parts of the tree, but this is not outside the realm of possibility. Other causes can be disease if there was a particularly wet winter in your area, as well as overfeeding if you have fertilized, recently. pH can be an issue if the tree is ONLY receiving bloom formula as a fertilizer. It needs a balance with as well, and will prefer the soil to be on the acidic side. Fortunately, we can rule out temperature over the winter. Yours happens to have quite the low temperature tolerance.

Here is an article for the general care of magnolia trees:

https://www.gardeningknowhow.com/ornamental/trees/magnolia/magnolia-tree-care.htm