Jan, my understanding is this. Tailwheel aircraft are generally much easier to land on grass because the little wheel at the back provides quite a lot of braking on the rough grass and the big tyres at the front tend to run over the grass quite easily. So it adds stability on the ground roll.
Transfer that to hard tarmac and the opposite becomes true, the big wheels at the front want to slow down quicker than the little trolley wheel at the back.
The net effect is that the back is much more inclined to want to be ahead of the front
groundloop here we come.
Increase the size and squishyness of the front tyres then you make the situation more exaggerated because on rough ground the front will skip over much more quickly but in return on the hard ground the front tyres will be very slow indeed.