This sort of thing can happen due to a disparity between the tyres of more than 3mm. The system detects that as slippage, engages 4WD when it doesn't need to be and causes undue wear. Buyers of cars with 'automatic' AWD systems need to be made aware of this. I've been trying to make them aware for a couple of years, but really dealers should be compelled to warn buyers before they drive away in the cars.