yes always photocopy any app, could be you've not sent enough evidence, but i think they would have told you to bring it with you, or they want you to clarify something, maybe you made a mistake , something conflicting that's why it would have been a good idea to have a photocopy so you could check it again

but more likely it's just a random interview, I've read of many people who had interviews, and most were given their visa after they finished the questions or a few days later