Oak trees produce acorns once per year. The number of acorns produced can vary from a handful one year to a large amount on what is called a "mast year." A theory that explains the excessive fruit production is predator saturation, when the tree attempts to produce more fruit than can be eaten by predators.
Attempts have been made to try and predict mast years, and records have been kept of when they have occurred in many species of trees throughout Europe. It is hoped that a potential trigger for mast crops may be discovered, although as yet none has been found.