It is often nature's way of reducing a heavy fruit load, explained Ross Penhallegon, horticulturist with the Oregon State University Extension Service.
Apples have several periods when fruit drop occurs.
The first fruit drop is right after the flower petals fall off and the fruit is tiny and may last two to three weeks. The dropping fruits are the ones that didn't get pollinated or the sperm cells from the pollen didn't make it to the ovary. Most fruit varieties need to be cross-pollinated by bees. Lack of pollination may be caused by cold or wet weather or honeybee decline. Also, if the flower buds are exposed to freezing weather, more fruit drop may occur.