Answer: The trees must bloom in order to set fruit, then the fruit must be carried to maturity. The most common causes of lack of fruit (assuming there is an adequate pollen source for cross pollination) would be frost damaged the blossoms or drought damaged the crop.
The pollenization could be an issue if there is no pollen source nearby (if for example it was a particular lone apple or crabapple tree that has been removed) or if there are no bees available to work the trees. Sometimes, too, rain or other weather conditions can interfere with pollenizing activity.
I suppose it is possible that deer would eat some of the apples, but you would see apples above the height the deer can reach. You would also see browse damage to the trees, especially during the winter. It is also possible insects or disease interfered with the apple crop.
Low fertility could interfere with the crop as well, but in my experience it would not cause trees to switch from a bumper crop to no crop at all. Some varieties will bear in alternate years, with a heavy crop alternating with a very light crop, so perhaps that figures into the situation.
Without knowing whether the trees set fruit and then something happened, or never set fruit in the first place, it is difficult to try to diagnose. I would suggest you consult with your local county extension as well. They should be familiar with factors that could have affected apple production last year, and they can also help you develop a routine care program for your trees.
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