My speculation on this is two fold. The first is that putting things in those surveys is a "permanent record" or at least "yearly review" issue for your manager. So you are (justly or not) harming them, and they will tend to respond to being harmed. The other is that, if you think about it... if your manager is not a person who you can talk to about a problem in a non-anonymous way... doing it anonymously is not likely to help, it's probably better to just quietly leave.