Stable coins will be de-pegged if assets they use to back their stable coins have troubles.
Troubles can be from their bad algorithm like Terra algorithm for their algorithmic stable coin UST.
Troubles can be from their shady finance management for their treasury to back their stable coins but unfortunately their shady treasury management is detected and leaked like FTX case that cause their token FTT went to the hell. That token is not a stable coin but it's good to use as an example.
Troubles can be from banks like bank bankruptcies