Your tree has gotten into a bad habit called alternate bearing. While it is not necessarily typical for citrus trees, it does happen occasionally.
The problem is the tree produces a crop that is too large and depletes its resources. The next year it does not have the resources to produce a crop, so it drops its flowers. But, the long growing season without fruit allows it to build up really large levels of resources. So, it produces an extra-large (too large) crop the next year. And, that depletes the resources – and starts the cycle again.
The solution is not easy. To break this you must prevent the tree from over producing. Next year it will set a large crop. When the fruit are about an inch in diameter, remove one-third to one-half of them. That’s the hard part. But, it will keep the tree from draining its resources. You should only have to do it once. The reward for your sacrifice will be more even fruit production year to year.
Consumer Horticulture Specialist
The LSU AgCenter and the LSU College of Agriculture