I have a controlling parent and the other parent is silent partner and will often take the controlling parents' side against me, and also against my sibling. I have been dealing with this for almost 10 years. And it does not stop when you move out.
Why do they do it? Because they think it will give them power to do it again. All that really winds up happeneing is that their so called "power" is drained, and they begin to feel bad about themselves, thus trying to reach you again in any way possible ignoring your boundaries and trying to control you again.
Some of these controlling parents will cover up their wrong doing to try and make themselves look good by saying that they are quote "doing what's best for you" - but in reality they just wanna see how much of your dignity and self-worth they can take until they can't take anymore. To read more and see other things that these sneaky controlling parents are aware of, visit these sites. They also talk about narcissistic parents, too.