This is an extreme case of fallen oranges, however, it is a natural event unless any form of disease is evident. The Navel Orange is a prolific in producing fruit, but if the tree decides it cannot service the whole crop it will drop some of the fruit. Also some of the fruit is known to split on the tree, this is also normal and the split fruit will also drop from the tree. Keep them cleared away to keep the fruit flies at bay.In most cases the larger the fruit the sweeter it becomes; if the crop looks like it is over cropping, thining out is the best answer.Now as for Lemon trees they look after themselves most of the time. My lemon trees flower and fruit all year round. I prune them to remove any deadwood and to keep some shape as well as keeping the height within bounds of my reach. They are fed twice a year with an occasional feed of liquid iron.
My figs are another story. I have two figs tree, one a green fig and the other a black fig.Both varieties grow a a fantastic rate every year. When I first encountered them they were at least 10 meters high. Now with the aid of a chain saw they are about 3 meters in height. They can put on 3 meters a year, if you let them. I keep mine pruned down to 3 meters because I can reach the crop at this height. Any higher and the birds eat the all high fruit before you can get to it. Keeping the boughs cropped back also encourages the tree to put its energy into making fruit and not wood. After all a large fig tree looks nice (apart from sucking your garden dry) but if you are not getting fruit, it’s a waste of space.