South Korea is bringing in new regulations for crypto exchanges from July 19th.
Exchanges will have to assess all tokens on their exchanges against new regulatory criteria. If the tokens don't meet the criteria, then they have to be delisted.
You can read the criteria here:
https://www.coinspeaker.com/south-korea-delisting-risky-altcoins/
They're going to have to do this every quarter - so an altcoin might pass this quarter, but fail next quarter.
Naturally this has caused panic selling of all alts, not just Steem and Hive. Some people think it's prudent to cash out now and then wait and see, rather than wait for a delisting, by which point the price will have crashed.
But the panic tanked the price of Steem and Hive anyway.
Update: it looks like both Steem and Hive passed the regulatory tests and remain listed