ふとした ... => unexpected / incidental / little bitty N2
Expected
~んがため(に)/~んがための      => in order to ~ N1
ずくめ => completely; entirely; nothing but ~ N1
ため => In order to, for N4
Purpose, target
ふそくはない => nothing to complain about, just as expected N3
As expected
ため(に) => For, because… N4
Cause and Effect
めったな => hardly; rarely; seldom N2
Ban
ずくめ => Completely A, entirely A N1
Degree/ Level/Amount
ふと => suddenly; accidentally; unexpectedly; unintentionally ~ N3
ふしがある => Seem N2
Speculation
めったにない => Rarely N3
Degree. Level, Amount
あらためる => to revise; to replace N3
Happens next