Personally, I've never seen this before. I don't know if these things happens from time to time or if this is truly the end of Steem and Steem Dollars
It's perfectly normal. Take a look at the price graphs of popular currencies like Doge, or Dash, or Blackcoin etc.
They all have an almost verticle spike as the coin gets discovered and the FOMO crowd piles in. Then a slow bleed as people take profits. Then some new coin will come out, and those who have held hoping for another spike get tired, sell and use the money to pile into the new thing.
The "new thing" in the last week or so has been Monero. It's not exactly a new coin, but the news is that some darknets are using it. Darknets used to use bitcoin exclusively and they played a big part in making bitcoin successful in the early days (payment on darknets was bitcoin's initial use).
That's why all excitable speculators are piling in. Of course to do so, they need to sell other stuff unless they have spare cash to hand.
So this is perfectly normal. At some point, there will be some big news about Steem, and the speculators will pile back into Steem. I've been in cryptocurrency since 2013 and I've seen multiple cycles of this.
it's addictive gambling.