How can there be an uneven number of sellers & buyers in the stock markets?

Not an unusual event. There might be 200 different people trying to sell a particular security and only 100 interested in buying it. The 200 will continue to drop the price at which they are willing to sell until the 100 think the price is cheap enough to buy all they have to offer. They might actually attract additional buyers as the price drops too. Actually, it takes only one buyer of a large amount of stock to drive up the price even though there might be a large number of sellers.