Its probably something you agreed to when you made an online purchase and took advantage of a special offer.   In the terms and conditions of the Special offer - you agreed to a trial membership program or something - even if you never read the terms and conditions - taking the offer was your approval to charge the same debit or credit card you used for the orginal purchase

This article explains it all  

Visa Tackles Scam Practices
(Wednesday, April 28, 2010)
Visa, one of the world's largest credit card companies, is taking aim at "scam" marketing practices that were quietly used by some of the Internet's largest retailers in recent years.
Retailers will no longer be able to allow third parties to charge a customer's card without the card owner re-entering credit card information, Visa said Tuesday. This is Visa's response to one of the biggest scandals to rock online retailing in years.
Last year, the U.S. Senate Committee on Commerce, Science, and Transportation launched an investigation after learning that thousands of consumers had complained about receiving mysterious credit card charges.
The committee concluded that millions of consumers were misled into signing up for so-called loyalty programs with the help of companies such as as Classmates.com, Continental Airlines, Priceline, Orbitz, Buy.com, and many others. Lawmakers said during hearings that these merchants had made an unholy but profitable alliance with one or more of three so-called post-transaction marketing firms: Webloyalty, Affinion, and Vertrue.
Under most of the agreements between the marketing firms and retailers, an advertising page is presented to shoppers while they complete a transaction at the retailer's online store. Many shoppers say they entered their e-mail address and pushed a large "Yes" button on the ad because it appeared to be a $10 cash-back offer or coupon. Many of those who complained say they thought they were being rewarded by the retailer for making a purchase.
Buried in the fine print are the full terms of the deal. Customers are notified that by providing their e-mail address they are joining a membership program and agreeing to pay one of the marketing firms a monthly fee, typically between $10 and $20. Many people said they didn't see this notice.
Visa's new requirement is designed to send a "clear signal to cardholders that a second purchase is being initiated and protects them from questionable marketing practices," the company said.
With the government leaning on them, many of the merchants involved have severed ties with the post-transaction marketers, which have also taken steps to alter their business practices. They haven't gone far enough, however, critics have said.

Greg Sandoval covers media and digital entertainment for CNET News. He is a former reporter for The Washington Post and the Los Angeles Times.