PUMP:

"The brothers are unsurprisingly bullish on Bitcoin. Cameron’s conservative bet is that its US dollar valuation will surpass $40,000 per coin — a potentially colossal figure — as he explained in a response to one Reddit user: "small bull case scenario for Bitcoin is a 400 billion USD dollar market cap, so 40,000 USD a coin, but I believe it could be much larger. When this will happen, if it happens, I don’t know, but if it happens, it will probably happen much faster than anyone imagines.""

DUMP:

"The twins are in the process of setting up the Winklevoss Bitcoin Trust, a listed fund to manage their Bitcoin wealth and bring greater legitimacy to the virtual currency. Papers for the initial public offering were filed in July, but the final decision is still pending. Due to regulations, Cameron was unable to discuss the trust in the AMA"

http://thenextweb.com/insider/2013/12/15/winklevoss-twin-says-bitcoin-valuation-will-top-40k-plays-down-silk-road-closure/#!p2vds

"In a move that could come to mark a major new era for the virtual currency, Tyler and Cameron Winklevoss have filed to float their stash of Bitcoins on a conventional stock exchange. The former Olympic rowers have lodged papers for a $20m initial public offering of the Winklevoss Bitcoin Trust, which holds their Bitcoin wealth, and would be managed by the Winklevoss' investment fund, Math-Based Asset Services. "

http://www.telegraph.co.uk/finance/currency/10157710/Winklevoss-twins-20m-plan-to-make-Bitcoin-mainstream.html

Obviously they want the price to be as high as possible, before they dump via this fund. This is a clever tactic, as the dumping via a trust can support the price (wow new trust, look at the adoption) and you can get rid of your coins. If you dump on an exchange it leads to total collapse.

If they are so convinced of this high price, why sell the position. Sounds like a scam to me.