http://thecryptosyndicate.com/bittrex-done-goofed-op-ed/
The above article theorizes that Bittrex offloaded their trading fees too quickly causing a drop in all their coins at the same time, that then propagated to other exchanges as an alt-wide panic.
Or someone else on Bittrex who has a massively diversified portfolio and all sold at once. Either way, it all started on Bittrex.
I believe what we are seeing is a market cap move into BTC prior to the upcoming forks. So much speculation that we will see another cash cow as what happened when BCH (BCC) resulted. I also believe that when the fork arrives, we will see boatloads of assets shifted back into the altcoin market. Be ready! Or, better yet... buy altcoins NOW.
That possibility was covered in the article but the data doesn't support it for this crash, it happened too suddenly across too many alts in unison.