General Grabber A/T Tyre Wear

[spooky_b329]

First post here so hello everyone I've got a 2wd campervan with Grabber A/Ts, its probably done about 15,000 on them. They are the Light Truck variant, correct load for the vehicle etc.

My question is regarding unusual (?) tyre wear. On the rear wheels (rear wheel drive) the tyres are evenly worn. On the front, the tyre tread looks stepped. I've not seen this before and the tyres appear to be in good condition, both front tyres are exactly the same condition, mostly road miles but I tow heavy trailers across grassy/muddy fields which is the reason I have them.

I'm planning on rotating the tyres but would be interested to know if this is common, and whether its a tyre pressure issue, a trait of of the tyres, or something else. The van tracks straight and apart from the usual shoulder wear from roundabouts, appear to be wearing evenly.

P.S Took me a while to get a decent pic and it has exaggerated the steps between the tread blocks, I've added a second image as well. I believe the Light Truck versions have a deeper tread than the regular sizes.