Why do tenured professors at high-ranked universities move to lower-ranked universities?
There are many reasons that people change institutions. I was tenured at an R1 institution and moved to a "high research" institution because I wanted to live in the area where the second institution is located. Although my current institution has a lower research ranking, it is the state's flagship school, where my former institution is not. So, I actually get more resources, better pay, and our students are more prepared when they start college, compared to my prior institution. So, your assumption that a higher ranked institution is better is not always true.