Horse chestnut tree
My horse chestnut tree is 25-30ft tall (not exactly sure how old.. I know at least 40 years old). Just this past week or two did it start dropping its conkers. Only problem is, they are "baby" conkers. Last year they were the normal size they always have been. Tree looks healthy.
The image does not provide much visually. It appears to be something in the crotch of the tree and some residue that is running down the bark of the tree.
This could be a canker on your tree.
Premature dropping of nuts can be due to a tree's physical condition. Disease, pests and insects can effect the tree nut production
Weather and environmental stresses can cause a tree to drop it's crop, this could include drought or excessive rainfall.
Poor pollination can also be a reason, and a tree can even drop nuts if there is an excessive amount on the tree.
I would inspect the tree for any other signs of disease or pests.