Would have to guess. But one reason could be institutional or hedge fund moving in or out. If this is happening, many times you may notice burst trades during the day.
Burst trades are just pre arranged trades which are timed and broken into 5 to 10 smaller closes. A way for big guys to move in or out of a position with stealth. Ever see 5 - 1,000 size trades in a row, several times a day, on the time & sales? That's burst trades, add them up and you know the original size of the big guy order.
Another way big guys do it is large volume at the end of the day or after hours. When their volume can't effect price.
The Idea (principle) is to buy or sell large positions at a given price, knowing increase volume interest will effect price. If they want the price to stay, they attempt to use one form or another with these stealthy tricks.
Your answer could be this or for other reasons. Maybe there just isn't many following the stock close. Or a trading house made a large bulk deal. Bulk deals usually can be seen in "T" trades being reported after hours.
They are hidden trades which just aren't reported when the exchange was made, but reported later.
At any rate there are a couple things which could cause large volume with little or no price change.
AI
