I think this is possible, thought BTS has the nuclear defense against it.
Step 1: Either install 51 corrupt delegates yourself somehow, or convince 51 delegates to collude. I think that trying to coordinate that many delegates would be hard and your plan would get revealed, but lets say you achieve it.
Note that if you are installing your own delegates, the fact that a ton of delegates have been replaced by new delegates is a big hint that something is happening, and should make people much more careful. If you do it over time, you have the problem of trying to maintain all these delegate identities.
Step 2: Rig the price feed of all your delegates, causing the blockchain to believe the price has crashed. Cover your bitasset shorts to make a profit.
The nuclear defense is that if you actually destroyed the system, the community will hard fork back to the old state, and eliminate the delegates (and possibly eliminate the stake which voted in all the delegates as well, if it was a case of one person voting them all in). There would then be two versions of BTS competing for network effect, and the community would need to align onto one of the forks. Of course, this would only happen if you did catastrophic damage.
I feel this is similar to 51% attacks in other coins. It is important that the community be watchful of delegates, they are the trusted party in Bitshares, we must be vigilant to make sure that delegates are very trustworthy.