What creates this "victim" mentality?

My ex has this thing where he seems to engineer situations in order to cast himself as a victim. For example, today I got an annoyed text from him because he had to do something with one of our kids he hadn’t planned to do; apparently he needed to cancel plans to get it done. Instead of explaining this to me when I told him what was going on, he nodded, agreed to do it, then proceeded to complain to me via text message. This is not a one off thing for him, he does it fairly regularly. What do you think the mindset is behind this type of behavior?