[CSX 4133]

Jim,
I doubt it was the braking, unless you already noted it pulling hard or lockup under braking previously. Is your car equipped with a brake bias adjustment? Like you summarized, not rev matching more than likely caused the hard clutch engagement and rear end to lose traction. Do you heal and toe routinely under braking/downshifts? Or are you simply blipping the throttle on downshifts where braking isn't needed?

~Steven