Last year ethereum also experienced a decline as it is now, but to my knowledge does the price not fall as badly as this? how could this happen? what is behind it?
I think mostly because of the whole market sentiment.
The main reason is bitcoin. It is having a bad year this year. That brings the whole market along. When bitcoin goes down by 10%, ETH and other altcoin could go down 20%. And the price goes down compoundly that way.