The afternoon plunge was affected by the Mt. Gox transfer:

Mt. Gox initiated a test transfer of 0.021 BTC.

A few minutes later, 526.9 BTC (worth $33.6 million) was transferred to an unmarked address: the address starting with 1HRApr.

Ten minutes later, M transferred another 42,587 BTC (worth $2.7 billion) to another address: the address starting with 18vjnB.

Then another 91,750 BTC worth $5.9 billion was transferred.