It's a combination of resisted motion and the Faraday-Lenz Law I believe.
I see the sample answers didn't explain the curved nature of the graph, although that is probably due to the fact that this actually would need calculus to explain, and that's not assumed knowledge for HSC Physics. For this reason, I think that you'd have gotten the marks even if you'd drawn the line as straight; the main thing they cared about I think was that it was initially steady, then decreased, and then steadied again.
The question would've been even more fun if the copper plate were circular in shape.