How does market cap of currency decrease? According to my knowledge, once a currency is mined, its already released so how does market cap change? Consider if Sathosi (the founder of bitcoin) sells all his 1 million coins, there will be people who would buy it. It would just be a transfer from one person to another. How would it affect the market cap?
The market cap is the product of the total currency supply and its instant exchange rate. If either the exchange or supply goes down, the market cap does too.

The supply generally doesn't decrease, except through things like burning coins (see this answer of mine for more information). The exchange rate of course goes up and down all the time.

Market cap is generally a pointless metric, because it does not in any way reflect the cost or income of selling or buying the entire currency's supply. Only small fractions of the supply are being offered on the market, so selling or buying any significant amount will easily move the price around.


  Consider if Sathosi (the founder of bitcoin) sells all his 1 million coins


Just to be clear: how many BTC Bitcoin's creator has is speculation.