I have never understood why people do this, but they send their money to an exchange and it can be a considerable amount and then just sell it all in one order. This means they get a lower price than if they sold it in smaller chunks throughout 1-2 days?
Just my couple of satoshi
some people are buying bitcoins not for the idea or concept but to make a profit. someone with...say a 10% gain strategy will sell at 10% no matter how far it may climb or fall. When they see a technical support they may buy all in and wait for another 10%.
They want fiat not bitcoins
i know because i used to do that and did well. I am in the bitcoin camp now though, Its not worth release my coins to fiat anymore.