No, it isn't just your wife. All I know is that the upperbody will jump-start the ticklishness, so to speak, and feet and other places get more ticklish as a result. :super_hap
Meka's body is the same way. That's why I save her feet for last most of the time.
If I completely exhaust her with upperbody tickling for 30 to 45 minutes, then her feet are so much more ticklish. She is scared when I go down there haha. She can barely take it.