Wondering how Opera got on my macbook

As I went to open a .torrent file tonight, something odd happened. Instead of Transmission opening up, Opera opened up. The strange part is that I've never installed Opera on my Mac and I don't see it in my Applications folder. Searching for it with Spotlight doesn't show Opera.app anywhere either. Anyone know how it got on my Mac or how to get rid of it? I'm a bit of a noob so any help is appreciated (had my Mac since Feb).

Thanks!