Mango Decline: root difficulties and resulting multiple deficiencies, and then susceptibility to any pest or disease that comes along, such as thrips, powdery mildew, and anthracnose. [I don't think I see any Mango Bacterial Black Spot on it.]
This is commonly caused by excess of compost, burying the tree too deep, or planting in potting soil that decomposes, settles, and becomes mucky.