More than likely, somebody got into the habit of lifting the griil by the handle after the casters seized.
EDIT: Something you might consider doing is putting a second screw in the expander to keep it from pivoting on a single screw. Any movement there will put added stress on the spot weld on the opposite side which is still rigid.
You may not need to be concerned about the end with the fixed wheels. On my Silver C, both left side extensions were broken from the PO using the grill like a wheelbarrow when the casters seized (that's my assumption). It seems like the natural thing to do. The grill was in such good shape I am certain it was kept in a garage or under a cover for most of its life.Not a bad idea but now that I know these extensions are vulnerable to breaking at the weld I may just add the nut and bolt to the other 3.
Its a very strong solution.