Here’s who (probably) did that massive $150,000,000 Bitcoin transaction

One of the unique things about Bitcoin is that every transaction on its network is publicly available for anyone to examine. Any time a user sends a payment to another user, that transaction is reflected in the "blockchain," a global, permanent ledger of Bitcoin transactions.
You can examine every Bitcoin transaction that has ever occurred at a site called blockchain.info. And that site says that a truly massive Bitcoin transaction occurred yesterday:

Those long strings of seemingly random letters and numbers are Bitcoin addresses. Each is associated with a secret encryption key that allows the owner of that address to transmit the bitcoins to another address. In this particular transaction, bitcoins from 15 different Bitcoin addresses were consolidated and sent to address "12sENwECeRSmTeDwyLNqwh47JistZqFmW8." The size of the transaction? 194,993 bitcoins. Given that one bitcoin is worth around $800 right now, the transaction is valued at more than $150 million.
Who was responsible for the transaction? I asked Sarah Meiklejohn, a computer scientist at the University of California, San Diego, for her thoughts. She's the author of arecent paper demonstrating that sophisticated analysis can reveal a lot of information about who is responsible for Bitcoin transactions. She has compiled a large database of Bitcoin addresses tagged with their likely owners.
While she says she can't be sure, Meiklejohn says that that 194,993-bitcoin transaction was probably done by Bitstamp, the world's second-largest exchange for trading dollars for bitcoins:

About half of the transactions sending bitcoins to this 12sENw address between August 29 and November 14 were from addresses we had associated with Bitstamp. This could be true for a lot of reasons (a heavyweight user withdrawing their bitcoins, for example), but there were a few other weird things I saw that made me think otherwise.
For example, a lot of the bitcoins that flowed out of the 12sENw address went to one of two other addresses: 1Drt3c8 and (especially recently) 1HBa5. The former of these addresses we have tagged as Bitstamp, and the latter is often within one hop of a known Bitstamp address (e.g., it has also sent a lot of bitcoins to 1Drt3c8).
So, while a lot of things could explain many bitcoins being received from Bitstamp, it seems like fewer of them could be explained by many bitcoins flowing from Bitstamp and then back to Bitstamp in a small span of time which is what leads me to think this is an internal shuffling of some kind.
Of course, I could also be completely wrong! For example, I should definitely mention that, for the direct transaction of interest, I don't have any of the input addresses tagged (i.e., they might or might not belong to Bitstamp), so that my inferences are really just going on the past behavior of this small handful of addresses.

So this probably isn't a case of one Bitcoin user sending $150 million to another user. Instead, Bitstamp was perhaps reshuffling its own funds, just as a bank might move stacks of $100 bills from one vault to another. Presumably, most of those 194,993 bitcoins belong to Bitstamp users who have deposited them with Bitstamp to facilitate currency exchanges.