Many people sell Bitcoin and crypto in general because there's still many doubts about it.
They are scared that crypto is a balloon that is about to pop any time soon and they want to secure their profits/investment.
Crypto is still in early phase and isn't mass adopted yet, so I understand where the fear might be coming from.
Yes, People fear that they loss all they bitcoin, if it will happen anything bad in the future for this cryptocurrency, so they go for exchange their coins to the local currency, But personally, I believe too much in this crypto, I always keep around 50-60% of my weekly income in my wallet and i use the other part, so If bicoin will banned one day, or a bad thing will happen, I say that i already withdraw some cash and i am not looser.