When the market is closed, the 'spread' is usually much wider. The buying price is higher and the selling price lower. However, in this case it seems like a real drop in the underlying share price. I have seen it go a half penny lower after close in the past, only to regain the half penny next day, but this is larger. Something afoot?
ETA further perusal shows 200,000 shares sold in about half an hour this afternoon in 3 sales.