This has been something that we've all noticed for Antoine's whole career. Even Tommy points out that Walker makes difficult shots look easy and easy shots look impossible. I have a theory that I thought I'd share with everyone.
First, consider that Walker is already an average-to-undersized power forward. Guy's like Rasheed tower over him. Second, consider that he really has no rise in his legs anymore. He was never a leaper and everyone loses some lift as the years go on.
So his whole post game is based on positioning, speed, and most of all, spacing. He has to expend a lot of energy getting guys off balance and out of their comfort zones. Slow guys he faces up on. Quick guys he backs down. A pump fake (or three), plus a spin or a lean gets him just enough space to get the shot off while still being cemented to the ground.
Now think about when you play basketball against someone who is taller and/or more athletic than you. You may know that you can get around them and get your shots in. However, its going to take a lot of energy to do so, and here's the key, you have to consider them even as you are taking the shot. That is distracting. Even though Walker is a professional, he's still not immune to this fundamental fact of the game.
Finally, I'll submit that Walker is a read-and-react player who needs to rely on instinct and feel moreso than a go-to move or a set play. That is why he's so versatile on the court.
So what does all this mean? I think he is simply thrown off his rhythm when he gets an apparently open (or easy) layup. He knows there are big men bearing down on him at any time and nervously puts the ball up as quick as he can. His feel is comprimised and his instincts are shot. Now when he throws up a twisting, lunging, fallaway prayer, he at least knows that he's done the work to get away from the defender. So he's able to focus fully on the feel of the shot. His playground instincts come through and he can make twice as many "bad" shots as anyone on the court.
So there you have it. My disertation on why Antoine misses bunny shots. I hope you feel enlightened by it.