Why'd Andrew suddenly dislike Bender's impression of Brian's life?

Because Andrew is a hypocrite. He is emotionally retarded. He knows the right thing to do but does not have the courage to stand by his convictions. Always going with the crowd. He only changes his opinion when others do. Thats why he got placed in detention to begin with. The ending gives us hope that misfit Allison will lead him away from this type of action.

It's in part due to his own strained relationship with his father, and in part to do with actually seeing Brian and his family as real people and not characters to be laughed at. It's one thing to laugh at something impersonal and in some ways imaginary, but when you start to realize these are real people, and then compare your own life with theirs, you sober up a bit. Of course it also acts mainly to foreshadow the reason Andrew got into detention eg. the kid he bullied and his father/son relationship.

Mostly I suspect the confusion in that scene is due to the performance of the actor because I had the same observation about that scene, but anyway..

My impression is that Andrew first enjoyed Bender's hilarious satire because, well, it was really hilarious but then realized that Brian wasn't enjoying it at all. This goes back to when Andrew bullied the kid in the locker room by taping his hairy ass cheeks together. At the time he did that, Andrew thought that was funny, but we see later in Andrew's monologue that he came to understand the humiliation his target suffered and now saw Brian as being humiliated at Bender's hands.

YMMV.

Edit: that scene could have been improved with editing. Right before he changes his reaction, there should be a shot of him looking at Brian and seeing him upset.