Be it an act of spite or desperaration or for whatever reason the writer decides.

The Dazzlings cast one last spell, one that turns the Rainbooms (Twilight optional) into sirens.

7466884 I rather like this idea, and it's different to most of what we've seen before. I'm a bit stuck on where the story would end up, though - either Equestria would help un-sirenify them, or they'd be stuck like that forever.

7467144
I'd have it so that they're stuck as sirens forever.

7467154 So the story would be about them adapting to life as sirens? I'm just wondering what else you'd explore with a sirenified mane six you couldn't do with just the usual three sirens.

7467309
I don't know, it's just something I would do.