The adversarial relationship between Nolte and Murphy, supposedly dissipated by the end of the first film, is revivified in the sequel via a couple of plot devices. (rottentomatoes.com)
There are two main stories really, one which sees Kronk back with «Yzma» again and the other revolving around his job as a camp counselor for the Junior Chipmunks, something that was touched on at the end of the first film. (rottentomatoes.com)
Not only that, but Shrek is a slightly different person (or ogre), having married Fiona at the end of the first film. (rottentomatoes.com)