The only other known incentive is arbitrage profit.
If shorts don't care to take a directionally bullish view on BTS, they can hedge it by buying the real asset with BTS on another exchange. Their incentive to go through this process is that if there is demand for the BitAsset, and they are able to short the MPA above its fair value, they make a net profit from this process. So in theory, if there are not enough BTS bulls shorting at the feed price, arbitragers can sit above that. In practice however, arbitrage is not simple for most people to do.