I see this pretty frequently - and it may be due to a number of factors - and sometimes a combination of one or more of them.
Drought stress
Poor pollenation, with these early drops being 'aborted' nuts.
Insect pests - walnut husk maggot, in particular.
Fungal disease - especially walnut anthracnose

Drought stress during nut maturation and poor/inadequate pollenation(cold, wet temps during bloom, or just not enough other BWs around to affect good cross-pollenation) are probably the two most common causes of nut abortion that I encounter.

I have had some droppage on my trees, and like Lucky says, we had a cool, prolonged spring followed by lack of adequate rains. We are still getting the occasional high forties nights, and sometimes it alternates with a mid nineties day. It's a wonder I am getting any fruit or nuts. They've all suffered. Win some, lose some. There are other crop years.

The exact same thing has been happening with my shummard oak trees. They have dropped very small and immature acorns, in amounts that seem to be what otherwise would have been a season's amount of normally mature acorns set to be dropped this coming winter.

There does seem to be an abundant amount of immature acorns still attached to the branchlets in the tree's canopy. That leads me to wonder if the tree over produced acorns, due to our two years of drought that have been more recently followed by four months of nearly daily rain events.Those trees must have produced so many acorns that they now must have been forced to abort great quanties of them, due to the excessive demand on the tree it would be to grow to maturity all of the acorns it produced during the years of drought.

The recent two years of droughts in our area were so bad that, last summer, all over town many 40-50 feet tall, previously healthy looking, oaks died. Even so, it is still hard for me to imagine how the water table could have dropped so low that even those deep rooted, mature, oak trees could not find enough moisture to survive.

I've noticed the same thing happening here. Too many good causes to focus on this year: drought, heat, a horrible spring, a plethora of insects. I haven't opened any of the nuts to see if they were pollenated or not.

Scott

I always anticipate a flurry of dropped/aborted nuts early on; it's a 'normal' phenomenon in my mind - and often, the first full-sized nuts that begin dropping in the fall are 'blanks', as well. That's why when I gather walnuts, I hull them right away and 'float' test them - those that float are inadequately filled, and I discard them from the outset.

I noticed the same thing happening on a Bitternur Hickory in a local forest preserve.

Down here in Maryland, we've entered a drought stage, and hickories were dropping nuts early this past weekend at a local park. Some of the oaks didn't look too great either, though I figure they'll survive, though they'll probably shed their leaves early this year.