It is likely an onomatopeia reminding the sound of a plane (Brrrr) followed by "Zin Zin", the sound of shots, falling bombs or the sirens used by the German bombers.

As others mentioned, this is definitely an onomatopeia related to big sounds, just like zinzin. It’s always hard to give an exact meaning to these words.

The way most readers would approach this is simply with the context. You know they’re describing something terrible coming from the sky, so you can imagine the word meaning "something happening which makes big sounds".

It’s so uncommon that even to a french person, it sounds exactly like this would sound to an english speaker: "From his brezinzin, he’s razing us!". In the end, it’s used to achieve some kind of poetic or literary purpose and it fits the tragic and fancy narrative trying to be conveyed.