For whatever reason there is a double later on that section of fender held together by some foam like adhesive, it traps water and hence the rust.
Only good fix would be to cut both layers out and weld in some new steel or do replacement fenders whether OEM or tube fender style.
Hers - 2011 Dodge Durango Crew
Mine - 2003 Jeep Wrangler X
1998 Jeep Cherokee Classic