Yoplait yogurt cups are designed with a smaller top than base. The top is just the right size to allow smaller wildlife to insert their heads to lick the residue in the container. When the animal attempts to withdraw their head from the container, the edge on the inside of the container catches in the fur, thus trapping the animal's head. They cannot get food or water while in this situation. Skunks in particular die from this because they cannot push the containers off of their heads.

A recent article explains it better: http://www.nhregister.com/articles/2011/09/16/news/doc4e72932405aee015982291.txt?viewmode=fullstory

Yoplait has resisted making significant changes to their packaging despite repeated calls from animal advocates to do so. Small wildlife pays a deadly price for Yoplait's trademark container.
Yoplait responds to the calls to change the packaging by putting a printed request on the container for the consumer to crush the cup. This is clearly not enough.
Tell Yoplait to stop endangering wildlife. They can do this by halting the use of this dangerous container design.