vendors account for the bulk of the stolen coins. Apparently the site made some feeble excuse and ceased vendor withdrawals for a week. That is a lot of escrow build up. Some social engineering with a trusted vendor or two kept things from going pear shaped too fast, and then they made off with all the escrow when the jig was up.
Before people say well it is a problem peculiar to pathetic junkies, the same thing could happen to anything in the BTC world that is very popular: An E-commerce site, a tumbler, e-wallet service, btc exchange, you name it. With sufficient incentive (a lot of btc in transit and a spike in price) the same thing could happen either on purpose or opportunistic or a hack, of course.
It was even more clever than that. When withdrawals were suspended, they announced that there would be a minimum withdrawal of 1 BTC when withdrawals resumed and put up a "countdown timer". So people with less than 1 BTC in their accounts who were silly enough to believe the bullshit about them implementing a tumbler actually deposited additional funds to their user balances so they could withdraw. They didn't just get the escrow funds - they also got a whole heap of people to add additional funds to balances they were keeping on the site (one quite weird thing which a couple of redditors have noticed is that whoever is doing this seems to like whole numbers).
And yeah, it's only the total losses which are larger in the drug marketplaces because of the sheer amount of BTC moving through it at any one time. People continue to lose money by giving their BTC to third party services, including some which are operated by anonymous players. There are multiple ways that user funds can be lost by any Bitcoin service which holds user funds, and that should never be forgotten.
The reddit chase is looking interesting at the moment. It's not confirmed by others yet, but it looks like the main sheep wallet already contained a
lot of BTC prior to SMP opening. That may or may not help in identifying the person behind SMP.
http://www.reddit.com/r/SheepMarketplace/comments/1rzfue/the_main_sheep_wallet_contained_millions_from_a/