The increase in bitAssets (bitUSD, but mostly bitCNY recently) is the reason for the recent growth in price along with the bullish alt market in general. When bitAssets are created, a short trader must lock up 1.75X the equivalent value in BTS in collateral. So, for every 1 bitUSD there is 1.75X that value in Bitshares locked in a smart contract as collateral and removed from circulation. The more bitAssets that come into existence, the less bitshares there are available to trade, thus increase in price.