It's similar to when someone immediatlely says, "I'm not racist, I have black friends", when they know they are at least a bit racist. they would never let their daughter date a black dude. Yet they immidiately go into 'don't judge me' mode.
It's about guilt.
Some people do change, but used to be the very thing they don;t want to be judged as. So if they have changed, they have sensitivities about being judged by what they did in the past.