User blog comment:MarineroLelouch/Power Ranking and Classes./@comment-92.96.32.189-20140215151816/@comment-7418318-20140215164204

That's one possibility. The other two would be that Minerva, after using her biggest attack, was too exhausted to redirect the attack.

Or that the weapon of the Nakagami set has the same "rule breaking" effects in the offense as it had in the defense, when Erza sliced Minerva's attack apart and thus can't be defended against with Minerva's magic.