why ethereum go down

1- ICOs - There appears to be a relationship between ethereum’s price and ICO sales – although the extent of this relationship remains unclear – it seems reasonable to suggest that the conclusion of a series of mega ICOs - which were paid for by ETH valued at hundreds of millions of USD - will result in some downwards pressure on ethereum’s price in the short term. The sponsors of these ICOs will naturally want to sell / hedge the value of the ETH they received.

2-
The most important reason as the rest of the currencies
Market correction. That’s absolutely normal and has happened every interval.

3-
Because a lot of people who invested want quick profits and when the price goes down a bit, there's panic selling

Finally :
Each correction period is accompanied by many news and most important that you trust the technology based on any cryptocurrency will not matter if you dropped because you know it is promising