I think it's a Yankee thing. I have no other explanation.
1952 Topps Mantle rookie cards aren't terribly rare. Yet their value doubles every few years. A mid range example that could have been purchased at any major card show in the 90s for $750 sells consistently now for $15k - $20k.
Mariano Rivera was one of greatest relief pitchers of all time. But his autographs sell for a multiple of any hall of fame inductee in the last 20 years. Ripken, Boggs, Gwynn, Griffey, Thomas, Ryan .. List could go on. Rivera sells for more than them all.
I guess there are just more Yankee fans with money to burn.