There's no one explanation for fast radio bursts. They are a class of things defined by their transient nature but from one to another their spectral appearance can be drastically different. Even from the ones discovered so far it's obvious they're not all one thing.

What remains the same is that they're from really far away and they're short. They can tell they're from far away because the broadband pulse has dispersed relative to frequency and the charges along the line of sight from us to it retard the progress of the longer wavelengths more. So it comes in like a 'swoop' of decreasing frequency on on the spectrum vs. time when reality it was more like a broad pulse.