Why? They are making money.
Exactly! At current prices, if I sold all my BC, I would be guaranteed to more than double my money (plus the ones that I mined that cost me nothing). If I hold for a higher price, I might make more, or it could tank and end up losing money. In any investment, there will always be those willing to sell low and make an easy bit of profit, there'll always be those who hold out for what they consider to be a fair price (and they may or may not get their price), and there will always be those who hold every coin they've got (or the vast majority) in the hope/belief that it will make them rich one day. It's all about deciding for yourself what your goals are and what your tolerance for risk is, and it is pointless to question or criticize other people's investment strategies. Just do what's right for yourself, what you're comfortable with, and to hell with what anybody else has to say about it.
Oops, I missed one... there'll also always be those who follow the 'advice' and FUD that other people spew forth and panic sell, often losing money and then getting pissed off at the world because they didn't take two minutes to use their head as more than a hat rack.
I'm personally holding, for now. I'll likely sell off a portion in the next few days, just to get my original investment back out and available for something else, wait a week or two to see what happens and then sell some more, if it'll get me enough to build another rig, and I plan on keeping at least 1/3 of my existing coins long term to be able to see personally and firsthand how the PoS system works and how it affects the longevity and value of a coin once PoW is done.