Was it Alonso they kept over Rizzo?
Combo of two trades when Josh Byrnes took over as GM:
12/17/2011 - Latos for Alonso, Grandal, Volquez
1/6/2012 - Rizzo for Cashner
Padres previous GM was Jed Hoyer who went to Chicago with Theo Epstein, and they had originally drafted Rizzo in Boston. So they jumped at the chance to get him to Chicago.
Unfortunately there was a thought with the Padres that Alonso might be better suited for Petco Park than Rizzo (ie, less power but more of a doubles/line drive hitter).