How is an spontaneous currency changing phenomenon called?
When the switch from national currency is to US dollar it’s called dollarization (this mostly happens for US dollar but there are exemptions like Montenegro using euros) and in general it’s called currency substitution.  It often happens when people lost the trust in the local currency or if it’s impractical to have local currency (for small states like Montenegro).

Past examples are Equador and El Salvador or Zimbabwe. This list is not exclusive but those are the major ones you want to look at. 

Also note that in case that people still use both currencies the currency substitution can be only partial but from my understanding Venezuela is close to full substitution.