The market value of a speculative commodity can move for any reason at all, as its entirely a consensus among the people buying and selling as to what other people will be willing to accept for it in the future.
> Large amounts of BTCs moving at the same time
The charts from mtgox seem to show that volume picked up after the price had plateaued and maybe dropped a little bit and then volume shot up further again after it went through a trough and a lower peak and started back down again, and once it got down around 200 volume dropped briefly and then picked way up (with a lot more price volatility) again when it dropped further to below 175.
Intuitively say that the first volume pickup was largely profit taking from the plateau and maybe the slight drop, after the slight rebound and smaller peak, I suspect the longer drop was panic selling, and right now (or, given the apparent delay, an hour or so ago) the high volume, high volatility oscillation is a combination of panic selling, bargain shopping for investment, and people taking advantage of arbitrage opportunities between the low price at mtgox and higher prices on other exchanges. But that's mostly just guesswork based on the look of the price and volume graphs.
The simple answer is that sellers kept taking the 'ask' prices on the exchange until the only ones left were significantly lower. See this graph of current 'market depth' (existence of bids and asks):
http://bitcoincharts.com/markets/mtgoxUSD_depth.html
In a double auction market, there are bids (offers to buy) and asks (offers to sell). The 'market price' is the last price at which a bid or an ask was taken by a market buyer. Taking a bid or ask removes that bid or ask, queueing up the next in line. In an illiquid market, there aren't many bidders and/or askers, leading to price volatility. In a liquid market (such as for US Govt bonds) there are many bidders and askers, meaning the price swings are much more likely due to a significant change in information.
Hence my contention that daily volume is the most important stat to watch on the BTC market. It's a proxy for market depth, which is a precondition for liquidity and relative price stability.
I guess most people were buying BTC because its value was rising really fast. Any price drop could cause them to lose their faith in it and want to sell them. And the drop (or at least growth end) was inevitable, because the price was rising faster than exponentially.
