I took this to mean JWs were reluctant to self-identify:
including people in that group not wanting to self identify to the government (see the video noted in the op as a reason why).
Sorry if I misunderstood your meaning.
I guess fear of the government is a possible reason for JWs not to answer the census. But you’d have to believe they were more scared in 2011 than in 2001 for some reason. Plus, while some JWs may be getting more scared, many others are probably growing increasingly cynical about Watchtower claims, and fearmongering, in view of scandals, overlapping generation, cutbacks, internal leaks on the internet, and so on.
A better explanation for the declining census numbers, I would suggest, is simple decline. Since it fits with reducing congregations and it fits with what many of us see in local congregations and conventions.