Price drop can be attributed to one thing - people taking out value from bitcoin. So there are people that sell all or some.
If they sold all, that means they don't believe in bitcoin as much as alt-coins, fiat, etc..., or are trying to play the high-low of short-term exchange.
If they sold some, that means the sellers realize the value, and will continue to hold their remaining btc, or sell later when they believe that bitcoin has matured further