I have never heard of the dispatch channels being encrypted. Even last week when they were looking for the guy from Oxford, the dispatch channels for I and G as well as the statewide comms were all in the clear. Maybe it just happened to go "digital" at that point?
Most likely it was just kind of interference but I believe they can encrypt but they never do,case in point when the sandy hook tragedy was going on a year and a half ago I monitored it all in the clear on a few of theirs channels and you would think that that situation would of been encrypted.although there are a couple of ids that are full time encrypted in the state police system.
With the understanding that I know nothing of CT government agency roles, my guess is that the DMV channel is related to commercial enforcement - trucks, cabs, or something like that and the big E is them being (overly) paranoid about nefarious drivers knowing where a checkpoint, sting, or impromptu checkpoint is being set up.
As I understand it, DMV enforcement uses secure when discussing things such as shipping manifests and other info that is considered sensitive. Less of a privacy issue and more of a way to reduce truck hijackings and trailer thefts.