Reports come out before the open or after the close, so the reaction happens in extended-hours and shows up as a gap. Options price an implied move beforehand (see straddle) and implied-volatility collapses afterward (iv-crush).
The stock's reaction depends on results versus expectations, guidance, and positioning going in. Good numbers can produce a drop if the bar was higher.
Example: a company beats eps estimates by 5% but guides next quarter's revenue 3% below consensus. The stock falls 12% the next morning.
Related: eps, guidance, earnings-call, gap, iv-crush