The best guess I have is that although our W's are ok with divorce and say it's over, they aren't really so sure. At least with my W I'm pretty sure this is the case. You are kind of a safety net for them, that should they change their mind they want to know you'll still be there. Moving on and doing things independently is a threat to that.
At the same time, it's also a threat to their decision. They have it mapped out in their head who you are and how you'll react. When you start to do things differently, they get ticked off. They don't want you to be different, because then they see you're not the same person they decided to leave and it makes them question their decision. It makes them angry because they don't want to rethink their decision, they want it all to go just like they planned. I'm not saying they sit around thinking about this, it's probably mostly unconcious thoughts they don't really grasp.
But that's just my analysis. I see the anger at this as a good sign. It shows that you are doing something that challenges their view of you. Yeah, they may get ticked off and feel threatened, but eventually they'll have to face that and start thinking about what they are doing.
