Essentially it is the 'wing root bending moment' that governs. We are required to load full wings and use ctre tank first. Thus the decreasing weight of the ctre tank fuel is constantly 'offset' at the root by the wing fuel weight. The wing root is stressed to carry this weight.
Now shut off the ctre tank and feed only from the wings. The weight 'carried' by the root is no longer offset by full wings and so ZFW becomes a consideration. This used to be a 737NG pre-mod problem in the days when we tried to avoid fuel frost icing and we had to then factor the 'dead' ctre tank weight into 'payload'. Essentially the fuel instantly becomes 'payload' in that situation although that seems a little bizarre! Of course the wing root issue would not instantly become a problem, but would grow as wing fuel weight reduced, becoming a 'max.' issue as you say, at landing. In the 'pre-mod' days we could alleviate this if we saw a problem by using more ctre tank fuel to bring the 'notional' ZFW into limit.