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This is common with heavily fruited citrus. They will drop some fruit early to focus on what they can. This will ensure that some of them will grow to full size.

Here is an article that is for lemon tree, but will also be the same for orange trees that will help: https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-fruit-drop.htm

This sounds like it could be a case of citrus brown rot. There is not a lot you can do for treatment, but if you use the methods that are described in the article, then you can rid the tree of the disease. Here is an article that will help: https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-fruit-brown-rot.htm

First, it is important to know whether this tree was grown from seed, or grafted. If it is grown from seed, then it will not likely be a tangerine anymore. Some citrus farmers will pollinate with wild citrus, which will show in next years seed crop. If it is grafted, then it should be just like the tree it came from.

If it has wild citrus genetics, then it is possible that it will never bear fruit. On the other side of things... If it is going to fruit, then it can be anywhere from 3 years and up towards 10 years before you even see a flower.

If you can include all information you can come with about the origin of this tree, then I can help you with much more accuracy.

If it is, in fact, a true tangerine then this article will help: https://www.gardeningknowhow.com/edible/fruits/tangerine/tangerine-tree-care.htm

This would be a BAD fungal/bacterial issue. You can attempt to fix this by applying wettable sulfur to the area around the tree, but sometimes this can be hard to control. This article will give you instruction on how to do this: https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-fruit-brown-rot.htm

There are many reasons for this to happen. It is necessary to fertilize these trees once a year in spring, and this is the most likely cause.

Here are two articles that will help:

https://www.gardeningknowhow.com/edible/fruits/tangerine/tangerine-tree-care.htm

https://www.gardeningknowhow.com/edible/fruits/oranges/no-fruit-on-orange-trees.htm

There are several reasons for this to happen. The most common is under fertilization, if it is not fertilized at least once per year. This article will help: https://www.gardeningknowhow.com/edible/fruits/oranges/no-fruit-on-orange-trees.htm