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.
What if nothing really happens on those dates?
The price of Bitcoin could just be manipulated by the people who want to implement an advancement to Bitcoin because they want to purchase cheap coins from the people that sell their coins. I would hold the coins and just wait for the price increase because theres no point in selling it right now if the money is only play money and fiat is pretty much worthless.