If you have the returner selected as your user controlled player but do not move him yourself the game will automatically have him try to field the ball presumably because the game thinks you want to attempt a return. A punt can go all the way out the back of the endzone and still your player will run towards it as if he is going to attempt a return.
If you don't want to attempt to catch the ball you need to run the return man away from it and don't stop movie him until the ball hits the ground. Otherwise the game may still move him to try and field it. You can also not select the returner and leave it entirely up to the CPU as to whether to try and field the kick or not. Generally they seem to make good decisions.
Probably the worst thing to do is to switch to the returner after the kick when the AI has decided not to field it. Doing so without manually moving him will cause the CPU to run your player back at the ball even after having moved away leading to a very high chance of a muffed punt.