It's called polyhydramnios, or hydramnios. Most of the time, it is of no concern at all. Sometimes, moms with multiples or with diabetes develop it, or babies with certain very specific birth defects. It has a number of causes. Overall, maniotic fluid and how it generates is very poorly understood.
about.com is a nice lay resource to start learning more. The first thread above has a link to the a gentlebirth article on polyhramnios, plus a homeopathic suggestion. In all probability, you might be measuring a little large, but your fluid is still pretty normal, so none of the delivery concerns will apply to you.