Some fans get engaged on stage between songs from their favorite bands, while some do it at meet and greets. This guy apparently heard "November Rain" and halfway through decided right then and there was the best time to get engaged, and clearly it worked!
Don't get me wrong – maybe the song has some special meaning to the couple, or they just love the song. I'm just saying it's an alternative to what we're used to in terms of these massive, grandiose gestures.