looks like leaves with slapper bars (aka caltracs, but the 'lesser' type the ones that have a rubber bump stop that hits the front of the leaf spring) not the 'better' ones that use a pivot and push down on the top of hte leaf in the middle where it would normally flex.
here's about the best I can do as far as a HD screen grab. It doesn't show the debris, but it does give the clearest pic (I went through a number of frames to find the best of what was there) of the rear end. I'd say it's a narrowed bw78, either from a commodore or a ford, but that's by no means definite.
Again - speculation - I suspect what has happened is the wheel nuts were actually tensioned too tight (maybe rattle gun, maybe just by hand) and that lead to them snapping under the added strain of the burnout. It'd be unlikely they'd snap if properly tensioned and doing that, since there's not too much weight in the back of a 1200 and it was in the rain, so the stress of the burnout by itself wouldn't be massive.
As I said though, it's only speculation, by no means is it an accusation, or a criticism or anything like that.
obviously click on the pic to see it full size