Immature acorns fall off oak trees for a number of reasons. Once the immature nuts are released from the tree, they will not mature, making them unsuitable for collecting for use as seeds or as food for wildlife.
Elements
Some immature acorns fall off the trees early due to natural weather elements. High winds can knock acorns off of the tree before they're mature. Heavy rain can also have the same affect, causing the acorns to fall from the tree before they've ripened.
Stress
When oak trees experience drought or other stressful events during the summer, they sometimes drop their acorns prematurely. The trees get rid of the acorns early as a defence mechanism. Instead of using up energy to produce mature acorns, the trees put their resources into surviving.
Pests
Sometimes the acorns are infested with insects, so they fall off the tree earlier than healthy acorns. Common insects that invade immature acorns include both lepidopterous and coleopterous pests. The filbert worm and the filbert weevil can destroy much of a crop if not controlled early in the development of the nuts.