This can be a bit alarming. We call this June Drop. It is just Mother Nature’s way of thinning out when there is too much fruit for the tree to cope with. The fruit that falls is know as Fruitlets and can look black and decayed. It is nothing to worry about, if fact, sometimes the fruit left on the tree increases in size and are better quality. I would recommend increasing watering to assist the fruit in growing larger and be able to swell. You may need to also thin out yourself by working through some of the branches and removing some of the smallest, weakest fruit, this will leave the better quality fruit.
It can also be a sign that the tree hasn’t been fully pollenated or there may be a lack of nitrogen, in which case, you should apply a general fertilizer, such as Growmore each Spring. Be careful not to over fertilize as an excess of nitrogen can, itself, lead to fruit loss. Another cause, can be competition for limited carbohydrates which is more likely to occur in poor weather with low light levels and cool temperatures. Warm nights may also cause fruit loss. If you have young trees it may be that the tree is getting established and it is too much for the young tree to cope with. I always suggest, if you are planting a year old fruit tree, to not let it fruit the first year, allow the tree to get established and eventually you will have a better crop. I also recommend, through the summer months, if we have a dry spell, to heavily mulch around the base of the tree to increase moisture levels.