Mites can lay eggs anywhere once they leave the enclosure. I've also heard that mite eggs can lay dormant until the right temp and humidity requirements are met, then hatch. So it's possible that the eggs came in attched to the bag or from something you bought a while back. The best thing that I can recommend is Provent-A-Mite. It is highly effective for treating infestations and good to use periodically as a preventative measure. You can order it here:

http://www.pro-products.com/