Because of the small size of these fruit, these drops are often overlooked. The fruit that fall at this time usually have not been pollinated, fertilized, or have been incompletely pollinated. In other words, there are too few live seeds in them to keep the fruit on the tree. They fall usually because of excessive competition among the fruit for the water and nutrients available to the tree. Excessive fruit set, or too little water and fertilizer, particularly nitrogen fertilizer, can result in a large second drop.
Some cultivars are also prone to pre-harvest drop.
There is not much the homeowner can do about fruit drops. Just keep the trees irrigated and fertilized and hope for the best. The calls on rhubarb are coming in now. I always get the first call of the season on rhubarb flowering from Great Falls in the second week of June. But a lot of folks encounter this problem.