What I think is causing all this huge dump in bitcoin is the pre segwit anticipation. Since a hard fork is happening, there are heaps of conspiracies of how this is when bitcoin will die. Then people are add to the drop by selling and rebuying when it drops even more
Then why all other possible altcoins are losing their ground too? Just take a quick glimpse here:
https://coinmarketcap.com/Besides some random occasional niche pump of 3 altcoins whole market seems to be crumbling.
This crazy decline can't be caused only by unclear situations of bitcoin scaling, BTC is simply not influential enough.
Alts are mostly traded against BTC, so when BTC falls, their USD price also falls, but if you switch to alts/BTC, you'll see that they have changed only a few percent up or down. Also, alts are in their own bear market for the last couple of weeks, and many of them dropped 2-3 times after reaching all time high. The most obvious explanation is that it was a bubble, fueled by ICO's and Ethereum. So, USD value of alts is tied to Bitcoins price, but their BTC value is more independent.