I think your theory makes sense but it would take a real knucklehead to enter a market sell of 400-500k shares given the volumes here. I think that number was probably lower and that the number of shares sold at the bottom was small. "Then the stock moved back up to settle where it did" and it did that quickly.
This wasn't a robbery....it was a mistake. And a smaller mistake than people are making it out to be. It's possible someone had just had enough.
“I have had a wonderful time but this wasn't it.” ..........Groucho