Answer: Even though you have two mimosa's, the growing conditions are not exactly the same in different places in your yard. The soil may be different, the site may be warmer or retain heat longer, you may not be watering at the same rate, etc., etc. Dropping foliage is a sign of stress. Lots of rain or wind, or even excessive heat can cause the leaves to drop. I'd give the tree the benefit of the doubt and expect that the stress is only temporary and it will leaf out again. I hope it does!
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