Question
4+ weeks ago planted 3 Tibouchina compacta, all in partial sun.  Am not familiar with this particular species, having grown T. granulosa, T. urvilleana.  Planting was before summer rains began and during severe drought, but plants were watered by hand and irrigation, flowered well, and appeared well-sited.  
Last week, after a good rain, leaves on 2 of the 3 plants (morning and noon light, planted within 4 feet of each other) began to curl under and then virtually every leaf dropped over 1-2 days.  No new leaves are appearing.  The other plant, which gets afternoon light, has a few curling leaves but no leaf drop.
Plants are in Ft. Lauderdale, FL.  Prior experience in micro-climate in Virginia.  Leaf drop looks like reaction to cold in Virginia.
Your advice will be greatly appreciated.  These are good-sized (3'x4') bushes and fill a key space in the landscape.  Thank you.

Answer
Hi Nancy, when a plant suddenly drops its leaves, it is more then likely caused by a change in the surrounding soil base effecting the roots; without seeing the plant and having a better understanding of its history it is difficult to pin point, but the advice I can give you is to check the soil base for anything outta sort. You sound like you have a good understanding of horticulture, so anything else you would have homed in on , so my guess is the root system has been compromised, but probably is reactionary and your plant should send out new growth, just keep an eye on the cambium layer, to make sure it stays viable. Nick