Central bank determined interest rates make it so that asset prices go up, in general, by default.

By having interest rates artificially low, it is much cheaper to buy assets on margin. Your assets don't have to rise as much in price for you to make a profit on your leveraged trade. That further causes more money to come in to make profit this way, which results in rising prices, which creates profit for the original traders.

In a normal market, shorts would keep this in check. Any time the assets get too overpriced, traders can reign them in with shorts, forcing leveraged longs to close and make a profit doing it. However this requires some kind of balance between the cost of borrowing money (involved in a leveraged long) vs. the cost of borrowing assets (involved in a short). When interest rates are set low by a central bank, it becomes cheaper to borrow money and more expensive to borrow assets.

Let's say there is an equivalent amount of capital going long vs. short at a particular time. And let's say the market doesn't move much after these trades are made, it's stable for now. The longs can wait out the lack of action. Their money is cheap, so they are not incurring major costs by waiting. By comparison the cost of shorts are very high, and they cannot afford to wait or they take a big loss. Rather than wait, they close, which allows the longs to profit by default.

Both longs and shorts as a result contribute to rising prices in this kind of market environment, where shorts should correct market excesses otherwise. Ironically the time when prices really do finally fall is when almost nobody is short any more.