Being the founder of a Bitcoin selling website I get many e-mails and reasons for selling. Of Course the latest reason for selling is people who bought or mined bitcoins back when they werent worth anything near what they are now are looking to cash in, especially at the Christmas period. We've also got a few bulk deals to buy a set amount from people who sell services and get paid in Bitcoin, especially with things like web hosting as they ultimately need to convert it into cash to pay suppliers.

Ultimately you decide when to sell. I still personally believe (perhaps its a gamble on my side) that its going to go up in value still.