Leaf drop from orange tree

1.5 metre high Orange tree in container (pit planted Jan. 1999) dropping leaves continuously with no apparent reason/visible damage. Leaves are pliable/healthy and smell of orange if rubbed but they are falling directly from the stem. Kept on enclosed, unheated porch year-round. Used an all-purpose pre-mixed liquid plant food for the first time in early May and the leaf drop started a week or so later. Flushed the container a few times since. Never had this problem before. Has never flowered but I don’t mind. Could the roots be burned? Does it need repotting? Should I cut it back?