Wikipedia
Biennial bearing (or alternate) bearing is a term used in pomology to refer to trees that have an irregular crop load from year to year. In the "on" year too much fruit is set, leading to small fruit size. Excess weight in the main branches can be too much for their mechanical resistance, causing them to break. Another major consequence is that flower induction will be lower, and the subsequent year will be "off" year (too little fruit).
The behavior could be due to plant hormones, particularly gibberellins produced in excess in the "on" years in the embryos of the young fruit. It could also be caused by depletion of carbohydrate reserves in the tree.
Biennial bearing is more common in certain fruit crops like mango, apple, pear, apricot and avocado, and is almost nonexistent in grapes.