This problem could be a simple as drowning if the plant as getting overwatered. Also check to be sure that there isn’t mulch piled up against the bark of the tree “volcano style”.  Both overwatering and volcano mulching can cause the symptom you describe.  Volcano Mulching will eventually kill the tree.  Get down and brush away any much until you find where the stem enters the soil and keep the mulch away from the bark a few inches.  It is also possible that it has a fungal disease.  I’d suggest bringing in some of the affected leaves to a local garden center or cooperative extension office to determine if that is the case