Yeah, it always amazes me how a market with a supposed 19 billion a day volume drops 8% with a few million dump.
Because it's a bunch of bots and children all wandering helplessly looking for a solitary stranger with a large and twitching bulge in his trousers to latch on to and go home with. It only takes one to form a lead and then everyone else follows in rapid succession.
This happens because there is a market estimated capitalization (this is when the average price for 1 bitcoin is multiplied by the number of circulating bitcoins) and the REAL capitalization - the price in purchase orders on the exchanges. Altcoins fall even faster due to the smaller support volume, Bitcoin is doing great.