I've also heard of "happy churn", where the customer no longer needs your services (e.g. when a dating site's customers get married).
involuntary can also occur when you cannot renew for technical reasons, such as credit card rejected. these aren't always fraud related, you can have someone who wants to pay you but cant anymore.
A: Voluntary churn occurs due to a decision by the customer to switch to another company or service provider, involuntary churn occurs due to circumstances such as a customer's relocation to a long-term care facility, death, or the relocation to a distant location.
from: https://en.wikipedia.org/wiki/Customer_attrition