not sure what could be causing this, did a quick Google search and only found one other guy with the issue. This is happening on all four wheels, no sticking brakes, all wheel bearings are good. Kind of at a loss as to why this would be happening. Possibly a wheel cleaner having a reaction from the inside? Mainly looks like heat spots to me though. Thanks for the responses!

 

That looks like the wheel getting hot cooking the caps...

 

What do you think would be causing it?

 

Either plastic being too thin and heat from the wheels is melting them, or a bearing or something has too much preload getting hot and overheating the wheel, but you should see some sort of discoloration on the wheel.... do you have a laser temp gun? If so go drive it and use the brakes alot and see how hot the wheel, rotor and if you can bearing are getting! Have actually seen a couple 4wds that the acle nut was overtorqued causing the bearing to overheat, even though there were no other signs, and it cooked the wheels and distorted the factory center caps. Have also see a couple bearings that when yiu first shake the tire top to bottom can feel play but when you spin the tire and try it again the play goes away, usually on 2wd trucks.