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.