The issue is cultural/physiological and related to the roots' inability to move water efficiently to the plant's distal parts - leaf tips and margins. The most common causes of this malady are over-watering, a high level of dissolved solids (salts) in the soil (can occur as a result of watering in small sips instead of appropriately flushing the soil when you water), and soil compaction, which limits both the roots' oxygen supply and soil gas exchange.

If you read this short piece which provides an Overview of Good Growing Practices, and compare it to your care regimen, it might give you a better idea of where you might be able to make changes to limit the stress currently affecting your tree.

Al