Most likely water run areas, where water is channeled to and runs down the side of the rig. That in turn causes any protectant you've applied, sealant or wax, to be stripped faster than the surrounding areas. And then UV will do it's job to oxidize those areas.
Buff, polish, apply sealant.
2020 Ram 2500 Laramie Longhorn 6.4
2015 Flagstaff 27VRL