Stocks Drop

Why do the stocks of certain companies drop when there is good news?
I’m very new to stocks but I’m noticing a trend with the ones that I am following. Why do the stocks of certain companies drop when there is good news? What are the plausible reasons? Am I not observing something?
The good news can’t beat the expectation of the people who bought the stocks, so they begin to sell the stocks. When many people sell the stocks, the prices will go down.
Stocks Drop Three Percent