I asked this question in a similar style already on bitcointalk and on bitcoin.stackexchange. But in these threads, there are no good answers and I thought maybe it would be better to ask the question in a PoW critical environment.
What can be done in PoW-currencies, if there is a majority attack aimed to bring the value of the coin down by somebody who holds a leverage short position? Or in other words: How can we trust miners, if they can profit more from doing strange things during holding a leverage short position than from the transaction fees? When leverage shorts exist, the attacker can leverage his profit, but how can security of the coin be leveraged?