My condolences to his family and friends. It sounds as though the gentleman enjoyed his car like most of us, tragic that it should end this way.
Sounds like loss of traction to one wheel, possibly caused by wet white lines/centre line detritus and probable excessive acceleration/deceleration and/or gear change. Suspension or tyre failure can also cause similar, but I suggest you dont ask me how I know that.
Sudden loss of traction to one tyre which starts to spin, the LSD then transfers power to other wheel which then, after getting some initial additional power down, also overcomes it's grip and also looses traction. At that point the entire rear is loose and the differentials in torque and grip start a spin. At speed it'll happen quickly and you'll be pointing 90 degree to previous direction of travel before you know it. MSP on these is good, but it's of it's time and it cant work miracles. Only way out is to catch it within the 1/10th of a second or so before it steps out beyond recovery; something you can only achieve with practice and even then it's unlikely if you're not expecting it (lewis hamilton spin leaving aussie gp?).
Modern electronic diffs and the latest versions of stability control are more likely to react quickly enough through braking, but most of the current ones tend to make cars feel less "responsive". I also wouldnt drive around with anything heavy and/or loose in the boot, it doesnt help. But the best answer to the problem is never to mash the throttle hard in anything other than good grip conditions. In low grip conditions the steering, brake and throttle use should be very smooth, both in application and release. On drying roads conditions can and do change very quickly.
Time for another wet handling day?