Sellers do not overwhelm buyers _when_ the price goes down, or vice-versa. There is no "overwhelming" of one group over the other. Nor do prices go down _because_ of an independent phenomenon other than buying and selling. The price is judged on the _value_ sellers and buyers ascribe to the stock at a given time. If they match, a transaction takes place. In the end there are always exactly equal buyers as there are sellers.
If there are 60 investors who want to sell stocks and 40 who want to buy always equally as much for simplification purposes and the price is $50, the price will keep moving down until there's 50 investors who want to sell and 50 investors who want to buy.
40 trades will happen on $50 price, but then there is 20 sellers still left who want to sell at this price. Since there are no buyers, price will go lower and slowly some sellers don't want to sell lower than $50, so there might be 15 sellers left at $45 and 5 buyers, they will do the trade and then there will be 10 sellers left. let's at $40 there will be 5 who decided it's good to buy now and this is where the fair price will have landed, at $40.
https://en.m.wikipedia.org/wiki/Order_(exchange)
For assets with high liquidity there will always be buyers and sellers, depending on how many are on either side and what their price limits are (if they set any) transactions will occur at different prices. Depending on the order size the price can vary even within a single order (if you need multiple buyers or sellers with different limits to fill it). The market price is just an indication of the current equilibrium price at which there is the most liquidity for an asset. If there are more people that want to sell than people that want to buy at a given price the sellers will have to reduce their ask price to close orders, that drags the market price down. The same dynamic moves the price up when there are more buyers than sellers.
