Corona virus affected US the most with their 5000 new cases per day and that has caused a lot of country wide panic that has crashed a lot of their markets. with that panic many bitcoin holders feared a drop like that so they started panic selling.
we all know that whenever bitcoin drops, altcoins dump 10 times harder which is why you see cryptocurrencies dropping this much. however COVID-19 has nothing to do with bitcoin which is why bitcoin price went back up.
and again we all know that whenever bitcoin price rises altcoins get dumped again which is another reason for altcoin drops.