A negative spread means that the bid price is lower than the asking price. This typically happens when there is more demand for the security than there is available supply, and traders are willing to pay a higher price to buy the asset. At such moments, it is said that the market is "tightening" or that prices are "trending higher." A negative spread can also occur when there is an imbalance in the order book, with more buyers than sellers wishing to trade at the current prices. This can happen because of several reasons. It can be a large institutional investor placing a buy order that sell orders can not match. This market is called "imbalanced" or "lopsided."