Why does the App Store want to know my billing information if the app is free?

On my iPhone 4s, I try to download an app from the app store, first it'll ask for my password which is normal, and I put it all in correct but then it'll redirect me to my billing information/payment method and ask me to fill it out. I can't remember what my prepaid security number is because I haven't had it in so long so I can't do it. But the apps I try to download are completely free so I don't understand why this would happen.

Is there anyway to fix this?

You have to have a valid account, you won't be charged but it still validates your account. Although I only ever just put password in so maybe something on your account has changed?

I'm having the same problem and don't know what to do?