They may not have gotten the product, or they weren't satisfied with the quality, or they don't recognize the charge at all. More often than not, however, the reason is "friendly fraud" -- The customer purchased and received the product, but claims they didn't so they can get their money back.
Unfortunately for small businesses, this is a difficult claim to defend, and often results in loss of time, money, and merchandise. However, having a knowledge of how the chargeback process works can help you find ways to defend yourself as much as possible from bad chargeback results. This infographic depicts the entire process, some tips for protecting yourself as much as possible, and also the consequences of doing nothing about them.
Infographic by: propay