According to the ALA, 7 out of the 10 most banned books in 2011 were written by women. Considering how female authors are left out of many literary lists, that's quite a surprising statistic. In 2010, 7 out of the 10 (mostly different) books were again written by women.
I doubt that any book banners are on a vendetta against female authors, attempting to remove all women's writing from their libraries. But there's definitely something strange going on here.
The answer, I think, is in the kind of books that female authors are more likely to write. 7 out of the 10 most banned books in 2011 had young female protagonists (8, if you count My Mom's Having a Baby). And at least 6 of them are marketed specifically for teenage girls. The others on the list are classics or non-fiction (or The Absolutely True Diary of a Part-Time Indian, which I must admit I know nothing about).
And in these girl-centered books, several themes occur again and again. Here's a look at the reasons these books were banned. TTFN series by Lauren Myracle: sexually explicit, unsuitable for age group. The Color of Earth by Kim Dong Hwa: sexually explicit, unsuitable to age group. Alice: nudity (in... a book?). What My Mother Doesn't Know by Sonya Sones: nudity, sexually explicit. Gossip Girl: sexually explicit. The Hunger Games escapes these accusations, but is instead accused of being anti-family and too violent. It is, notably, the only book on the list criticized for violence, despite the fact that there are many equally or more violent books available on the shelves and marketed for teens.
Could it be that, frequently, banned books (especially banned YA books) are less about protecting the population in general, and more about preventing young girls from accessing "harmful" ideas, especially regarding sexuality? Is it that these things seem worse, somehow, when the protagonist is a girl? Whether or not it is intentional, banning books seems to have a harsher impact on female readers than male readers. Female readers are more likely to pick up young adult books (which are, in turn, more likely to be banned). They're more likely to find character like them labelled "unsuitable" and find that many issues that are relevant to them, such as female sexuality and female struggles in general, are considered "off limits."
And that is why book banning is a feminist issue.