Patrick Mahomes, by most any reasonable measure, was the NFL's best quarterback in 2018. As such, he's a good candidate to decline in 2019, because when you start at the top, there's nowhere to go but down. Even beyond random fluctuation, though, there's reason to think Mahomes wasn't quite as good as his numbers looked last year, and that he's unlikely to get the same breaks this fall that he did back then.
Derek Carr, on the other hand, was a bad quarterback last year, but he too may have played worse than his numbers would indicate.