App Store asking for credentials?
This appears to be a recent change to how purchasing functions, added for increased security. According to TheNextWeb, via MacRumors:


  In the past 24 hours, Apple appears to have started prompting iOS
  devices owners and those with Apple IDs within iTunes to make their
  accounts more secure, requiring them to pick three security questions
  and enter their answers when they download a new app. 
  
  The company is also asking users to enter a backup email address, in
  order to better protect their device but also their account (which is
  tied to Apple’s Retail website and all of its media services).