There are several possibilities but all are related to root stress. Transplant shock from damage during planting (such as damaged or drying roots) is one possible cause. In the summer heat transplanting is a more difficult event for a new tree or shrub. Another thing to consider is if you used fertilizer in the planting hole this can burn roots. There is little to do for that except to try and give the plant a good soaking to dissolve and wash away the fertilizer salts. Just don't keep it too wet. This brings us to a final thought which is that if you water too much the soil gets soggy and roots lack oxygen. This is deadly to plants especially in the summer heat. In your area many of the soils are clayey and thus drain poorly making such a problem more likely. If you can identify which of these is most likely and then take steps to prevent the problem from continuing the mimosa will have the best chance of recovering. Good luck with your new tree.
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