Good discussion. I think an average autox pavement has a lot to do with it vs say: tire choices. So it sure seems that either bumps and/or very quick transitions between rear wheel spin and hard brake applications are the major cause. Why technically speaking i have no idea, except that is surely looks that such instances make ABS engage to prevent lock up, yet be unable to engage just yet, perhaps due to the car being unsettled.
Interestingly, even pros point out to bumps and street circuits in general (due to bumps) as having to be treated very carefully and GENTLY under braking. I know i have said it before, but the initial (softer) application (even if only for a nano second) to settle the rear before standing on the brakes is crucial. It is also much harder to accomplish (well) vs just say it.
Here's some general thoughts on late braking from the pros. Note - specifically - what is being said in the first 1.5 mins of this quick vid. This is not at all unlike the problems that pros in race cars can experience in similar situations.
http://safeisfast.com/video/late-braking/