You're right. There are some people in the trading platform who doesn't know what they're doing or maybe they have their own reasons why they're selling their bitcoin when the price drops so here are some of the reasons i know why they're selling their bitcoin.

1. Panicking - They're panicking about their lost. This is the most common mistake of the newbies in the trading platform they're panicking and selling their bitcoin when the price drops.
2. To cut their loses - They sell their bitcoin to cut their lost. They don't want to lose a large amount and choose to sell it right away while the price is dropping low.
3. They don't have enough knowledge - They don't have enough knowledge about bitcoins price, If they don't research about bitcoin they will think that it will not going to recover. And ending up selling their bitcoins.

All the reasons i stated comes from my observation, threads i read and my own experience.