“Meta just got hit hard — and it’s not because of weak sales. Here’s what scared investors.”

The recent sharp drop in Meta (META), the main reason is legal risk.

Meta is facing a major U.S. trial over allegations that Facebook and Instagram were designed to be addictive and harmful to children. (Reuters +1)
The states are seeking potentially huge penalties/damages, creating uncertainty for investors. (The Wall Street Journal)
META fell about 4% on Tuesday, Aug. 18, making it the biggest decliner among the “Magnificent Seven” that day.

Investors are therefore pricing in possible fines, higher legal costs, and changes to Meta's products/business model.

In simple words:
Investors are worried that the child-safety lawsuit could become very expensive for Meta and force changes to Facebook/Instagram.