I am new at growing citrus, too, but my Navel Orange tree sets a lot more fruit than it can support so about 75% of them turn brown and fall off, but that leaves all the tree is big enough to support. Examine the fallen fruit and the branches to see if you can find any signs of disease. If you find any signs, then you can google pictures of citrus disease.
Your tree looks very healthy, but perhaps you should stake it straight and encourage one of the branches to form a central leader. That will not make any difference with the fruit dropping, but it will help create a stronger tree.