Also keep in mind that they can't cash out their collateral until someone sells HEROs back to them. The only way it could all be rapidly dumped is if people were holding both sides of their short positions. If we actually get real adoption by long term HERO holders, then it will be a large market cap market but there won't be enough HERO liquidity for everyone to destroy their HEROs at once. If that happens people shouldn't all want to sell at once, either, because it will be a healthy and growing market that provides real utility.
If somehow a few people shorted HEROs to themselves while pumping the reported BitShares price up to the target price with fake volume somehow and won the prize, they could all then dump their collateral and crash the price.