Explanations for many challenges/bans are located here.
The ALA also has a page containing a summary of "Challenges by Initiator, Institution, Type, and Year." It doesn't address any of the individual titles, but is good if you just want an overview of why/when/where/by whom books are challenged.
[And for any who don't know why this lemming is going around, Banned Books Week is this coming week, Saturday to Saturday. My lemming-ness regarding this list is located here.]