How exchange platforms work? I am wondering how exchange platforms such as Kraken or Bitfinex can survive bubble exposions? People exchange real currencies for cryptocurrencies. When a bubble explode a lot of people want to trade cryptocurrencies for real currencies so the exchange platform will lack liquidity. 

How do they can survive and avoid bankrupt?
This is not really a bitcoin related issue.  As long as the exchanges have 100% reserves, they can always pay.  This means they need to have the sum of all balances of all customers on their own account.  

The problem is that some exchanges apply a fractional reserve.  Using part of the users money to make investments or to cover operating costs.  You are correct to be worried for those exchanges bankruptcy should exceptional selling occur.

Kraken has a concept where they try to cryptographically prove their reserves https://www.kraken.com/security/audit

Not sure if other exchanges also have such a concept.