You're halfway right; the Russians have observed the same situation using their wheeled BTR60/70/80/90 8-wheeled personnel carriers for transporting ammunition and heavy supplies in areas where overhead protection from artillery airbursts and chemical warfare agents-= their own-is critically important and cargo trucks can't be used.
So laden, the vehicles do have a lower center of gravity, making them less prone to longitudinal rollovers [though bogging down in sand, mud or snow is worsened] But once the vehicle begins that rollover process, the extra weight increases the inertia to continue the effect, and once the critical angle from which recovery is impossible is reached, over she goes.
In the case of the Stryker's RPG screen, it *might* help serve to prevent a full rollover on pavement or hardpan stone or clay, if probably not in sand, much like the rollbar of a racing car or the ROPS cage on the old M151 series jeeps.
Or it may serve as a choppper to dice any crewmembers thrown out and off, or any dismounted bystanders nearby when one goes over. It also appears that the 4-wheel front steering is a contributory factor in Stryker rollovers. And there doesn't seem to be any easy way around that.