apples dropping from our tree?

We have a lovely apple tree in our back yard (two, actually, one of which is small and doesn't produce fruit but I think serves to help fertilize the larger one). The larger one has countless apples growing on it. But they drop by the dozens while still small and inedible. It happens even when there is no wind and no squirrels seem to be running about. Does anyone have any ideas why this happens and what we might do to prevent it?

thanks.

How close are you to a main road? The exhaust from passing cars could be stunting the trees growth and causing the apples to drop prematurely. This happened to the tree in my grandparent's backyard (Washington State, eastern part).

Could it be a crabapple tree? Did it ever produce full-size apples?