I think it can be quite easily explained as a musical interpretation of events that transpired. It happens all the time in the real world, after something major occurs. For example, you got all these songs popping up about starting revolutions when the revolution in Egypt occurred, and shortly thereafter when America was teetering on the brink of a possible revolution, as well. It got close enough that there were meetings with upper-level military officials to discuss what would happen if that did go down. Then, all these songs about it popped up. Think of it like that. The WoL defeats titan? The magitek nerds make a tribute to what they think slaying titan might sound like, from an artistic approach. Consider the songs as musical tales of exploits done by the WoL and other famous heroes. This has been a constant theme throughout the history of music. It's typically inspired by current events. And remember... This game is far more steampunk than it is fantasy. So, some of the music you may think is out of place cuz it doesn't suit a Tolkien world could very well exist.
