You don't get how cashing out 50x340 bitcoins at once in 50 accounts to bypass transaction account limits to withdraw 340,000 dollars at once and more other the next few days could possibly crash the bitcoin market?
Ok, newbies, calm down.
Deep breath.
Those coins in the LulzSec tweet have not been spent (ie. not sent to mtgox etc.). The coins have not been sold, therefore those sales did not crash the market.
Either they are taking credit for someone else's coin transfer, or they hacked into a miner and stole their wallet.