According to the filing the sale was below market price, at $13.81 a share, "in a privately negotiated block trade." At the time I write this post the stock is trading at $13.93. As recently as last week the stock price traded at above 16 dollars a share, but that was before B&N revealed that they were under SEC investigation.
While it would be fun to speculate as to why Riggio sold off the stock, I think I already know why. Remember earlier this year when Len Riggio announced plans to split B&N and buy the retail stores away from rest of the company?
At the time he revealed that he had acquired additional stock, probably as part of a plan to force a proxy fight and split the company. Given that we know his plans to salvage the retail stores have been cancelled now that he leads the company again, there is little reason not to sell off some of the stock he acquired.
Update: Barron's scored an interview with Riggio and he explained the sale:
Riggio explained that he owned high-cost Barnes & Noble shares purchased above $35 a share. The sale of the two million shares established a tax loss this year that can be used to offset any gains that he may have realized. Riggio presumably also owns a lot of low-cost Barnes & Noble shares.
The 72-year-old Riggio said he made the decision to sell the shares last month after consultation with his tax and financial advisors. "I don't even follow the price of the stock. I never looked at the stock price when I was selling."