I have experienced this, being very subservient to people who have hurt or mistreated me.

Possible reasons I have wondered about:
1. Seeing relationships as a game or power struggle. If the other person comes out less hurt then in a way they have "won", and that can make them seem to us very powerful and admirable in a twisted way.
2. A minor version of Stockholm Syndrome, in which someone hurts us and out of fear we become overly attached to them and ridiculously grateful for small kindnesses.
3. If you are very angry with someone, but you not a person who is able to naturally express anger , then the anger comes out in strange ways such as being overly nice to the person. I think there is a therapy term for this kind of phenomenon but I'm not sure what it is.

I think for me its a bit of a mixture of those three.