Not really sure how all these stable coins work but how did a stable coin that is supposed to be equal to one US Dollar go down in price? 
USDC is made by Circle.Circle has $3.3B deposits in Silvergate bank.
Silvergate bank files forced liquidation and that means a lot for Circle and mostly affecting USDC.
See news: Crypto bank Silvergate announces liquidation amid sector turmoil
Just like the collapse of USTC and Luna, that means a lot to the market which affects the entire whole crypto market. And whenever there's sort of news like this, the involved crypto whether it's a stable coin or an alt coin, it's always been affected and getting a lot of dumps which results to its value goes down.