Hiya @Lowe .
There are many reasons as to why this could happen. Firstly I usually check for pests and/or diseases. Secondly, has the weather suddenly changed?
I've got a potted citrus tree, and I've learnt that they can drop leaves during heat and/or water stress.
1. Weather. Citrus may dump their leaves in very hot or very cold weather. They will grow back if this was the cause.
2. Fungus. If the leaves are turning yellow, feel mushy and have brown crusty edges then they have a root fungus caused by excessive moisture and lack of root oxygenation. Application of Copper Sulphate to the roots may help fortify the plant against the fungal disease. Drying the soil by refraining on watering may also help.
If any branches are dying then look into phytophthora. Even one or two random dead branches can indicate this disease. I successfully treated it with Gogo Juice from Neutrog. I am going to get Popul8 which apparently fights it even better.
If the branches are healthy then I'm going to say it is the weather... you can have lots of rain then the horrible dry wind that will suck up that moisture and your leaves can drop.