Originally Posted by Andrew R Stewart

Agree that bending forces contribute to broken bolts. There are disk brake specific racks that have a built in "stand off" to provide strut/caliper clearances. These reduce the bending forces and make the bolt work in shear more then a bending way. While SS can be weaker then alloy steel it's when corrosion resistance is needed that SS is best. Although I admit to having to deal with fewer "rusted in place" rack bolts then "allowed to be loose and bent till failure" bolts. Having both happen at the same time really suggests that the bike's maintenance is lacking, for the manor the bike is used.

25 lbs on a rack 5 or 6 days a week for months/years adds up to a lot of stress cycles and not a small stress at that. Can you mention the rack brand/model and whether it has any triangulation in it's lateral structure. (As example some racks have a pair of struts angle inward to the top to better brace against side sway). I see bikes with flexy racks and big loads too often. A flexy race will stress the mounting hardware worse then a stiff rack does.