According to University of California, the falling of immature fruits may happen for several reasons
Natural: Generally the avocado tree produces about one million flowers, but only few set fruits (may be 100 to 200) but the remaining pea to walnut size fruits will drop off.
Water stress: Application of excess or too less water causes stress leads to fruit drop.
Fertilizer: Excess application of fertilizers, particularly nitrogen may cause this problem. Avoid application of nitrogen fertilizer from April to mid June.
Insect damage: Insect pest like thrips may cause fruit drop. This insect sucks the sap from leaves, stem and fruits. So inspect the fallen fruit under the calyx for any thrips.
Also, according to research in Israel, the flower pollinated from pollen of other avocado trees has tended to stay in the tree and set fruit. So planting two or more avocado tree together helps in reducing fruit drop.