It is not rocketscience.
People sell because at the moment DRK and now also NautiusCoin are way more profitable.
And major dumps for BC are still on-going, no end in sight. Holding BC is not smart at the moment.
In fact it is. I fell into this trap more then once. Some other coin is pumped and you sell some good coins to buy into the other pumped coin. Then the other coin is having a correction to a much lower price. After this BC will rise again and you lose you money. Your new coins are worth much less then you bought into. And the original coin (in this case BC) is worth more again so much loss ;-)