It could be anything, Stripe will tell you everything they know from the user's bank. Most likely the bank thought it was fraud (for charges way more than usual, in different countries, etc.), they've not got enough money in their account, or perhaps people are carding[1], which will generally happen if you charge a very small amount that people are unlikely to notice missing.

[1] http://en.wikipedia.org/wiki/Credit_card_fraud#Carding