I'd say it's involuntary if you don't have an option (i.e. if you don't accept the downgrade you're not getting on that flight).

I'd say voluntary would be if you asked to be moved to a lower cabin because you weren't happy with the seats available (e.g. some people would prefer upper deck in CW than middle seat in F, or an exit row in WT than a middle in WTP).

Sometimes people ask to be downgraded if a friend/colleague/partner turns out to be on a flight but in cheaper seats and they want to travel together.