It looks like all altcoins dump 10-20%. What is the reason?

I am very sad

The reasons are varied. But what is most important to know is that you shouldn't be feeling sad at all. Dumping and pumping are two very elementary things that happen here in the world of crypto. There's fluctuation that happens no end. So, sit back and keep calm. After it falls, it rises. That's certain.

This is the biggest test / tragedy for traders, I see the cause is a hardfork problem in August. Everyone is worried about the impact. They make a panic selling all the bloody coins. Wait until this storm ends.