There are several possible causes for leaf drop. Be sure that you are not watering the area around the tree any more than once a week. Established citrus trees are rather drought tolerant and frequent watering is a possible cause of leaf drop. Also, do not use any "weed and feed" products near the tree since the herbicide that is usually used in them, atrazine, can be picked up by tree and shrub roots. and cause injury.

Also check the base of the trunk (the lower 2 feet) of the tree for evidence of peeling bark. This can be an indication of foot rot disease. Foot rot disease can eventually partially or completely girdle the trunk by killing the cambium, the thin layer of green tissue just under the bark. This can cause the decline or death of the tree. The symptoms can include leaf and fruit drop, leaf yellowing and dieback. Excess moisture around the base of the trunk of the tree can encourage foot rot disease. For this reason it is best to keep mulch, weeds and other vegetation away from the trunk. If there is peeling of the bark part way around the trunk, an application of copper fungicide after removing the loose bark will often arrest the disease.

Additional Resource

http://edis.ifas.ufl.edu/pdffiles/PP/PP26200.pdf