its the tyres
they are old and volcanised and offer no traction. as soon as the car starts to rock forward and brake hard the rears just start to slide.
my remingtons from the wreckers on the rear of my ke70 do it all the time. put decent tyres on the rear and it wont be an issue

you'll also find its very easy to lock the tyres if you are braking, drop and gear and lift the clutch the rears will lock.