It’s sun fading, almost certainly. These, like all mass produced paper printed products, would be litho printed in CMYK (Cyan, Magenta, Yellow, blacK) process. Of these colours, yellow is most prone to fading from UV exposure (followed by magenta, black and cyan are more stable). So when that orange colour, made up principally of yellow and magenta tints loses some yellow, it goes pink like that.
Yes, you do get variance across a print run, for many reasons which are too boring to explain, but the print operator would (should) flag these in the stack of uncut sheets, same as with the many other possible print defects that can occur in a print run (especially if the operator is not particularly attentive). These days the litho presses have scanners that automatically correct inking levels rather than the operator needing to check levels constantly which makes things more consistent, but it’s still not 100% across a run. Rogue sheets do slip into subsequent production processes, of course, like any manufacturing process you get a small percentage of defects. But it’s unlikely you’d get something that varied that much make it all the way through the quality checks.
So I’d go with sun fading.