Below is the order book for bittrex, which has the highest volume, as you can see the sum of all the bids is only equal to 14.248 BTC which is currently worth around $10,000. Unless there are some hidden orders in there, someone could send 14.248 BTCs through and cause a flash crash wiping out all the bids.
Actually, they can't. There is 14.248 btc in the orderbook, but all of that can't be bought on the market price. The sum is 1 0966 480 steem, and that is much bigger than all liquid steem. So it is true that a whale can bring the price down significantly, but it's impossible to bring it down to zero.
A good strategy for someone receiving a payout would be to hedge against a potential fall by going short Steem prior to the payout and continue holding short, cashing out their Steem Dollars into Steem and then selling Steem into BTC. However I don't think this is possible, on poloniex you can short a few of the crypto currencies on there, it does not look like an option for Steem at the moment.
If somebody thinks that steem price is going to fall, and there is no possibility to short it... then what's the next best option? Of course to exchange it to either fiat currency or to some price stable cryptocurrency. And guess what? Steem dollar is ideal for that!
So it makes sense to save all your SD if you think that steem price will go down. And on the other hand, if you happen to have steem and think it might go down, it makes sense to buy SD with it. That can be done easily and for free in the internal exchange. This has one really big advantage: the money stays in the ecosystem. Users don't need to sell it for bitcoin so it doesn't affect the price.
And then there is the interest. It can be adjusted by witnesses, but apparently it's starting with 10 % which is huge! Make easy money just by holding SD. I wouldn't be surprised if there was big buy pressure for SD because of this.
One really interesting piece in this puzzle is the liquidity reward. It should make sure that there is always at least some liquidity in the internal market. That is a strong force to keep the Steem dollar worth one US dollar.
July 4th is big unknown so probably there is a lot of volatility. It means that active traders will have a lot of arbitrage opportunities.