I knew he would lose minutes to Big Baby upon Baby's return, but losing minutes to Scal? I thought with Baby back, that would be the end of seeing Scal. I thought we'd see the Baby/Shelden tandem. Scal played 10 minutes and had 0 points and 0 rebounds. Shelden could have done that. 
I wonder if Shelden is in Doc's doghouse for some reason. Has he not been practicing well? Is he fumbling passes in practice worse than he does in games? Anyone have any idea what's up?
Well, Scal was playing those minutes at SF, and Shelden cannot play SF. Scal's minutes say a lot more about Walker and Giddens than they do about Williams.
But to answer your original question, I don't think Williams is in Doc's doghouse, I think he just is not that good of a player. Seeing Davis, who is a mediocre player himself, play signigicantly better than Williams has all year, after practicing just 1 time in 2 months shows just how mediocre Williams really is (not to mention, how terrible guys like POB and Mikki Moore are, who are significantly worse than Williams).
Williams has done a nice job filling in this season, and he is a valuable guy to have at the end of the bench, because he is capable of filling in without killing the team. However, he just is not a rotation quality player on a championship team.