I see what you mean. I’ve had this case for two years now, and honestly, I didn’t even notice the yellowing until I read your question. It seems to be mostly around the edges—probably where I usually hold the phone. My guess is it’s from the oils on my hands staining the plastic over time. I’m no expert, just using common sense here. I think the only way to completely avoid it would be to never touch the case, which obviously isn’t realistic. That said, it doesn’t bother me. It’s really subtle, and for a two-year-old case, I think it’s held up pretty well.